unable to reinstall ubuntu

Can anybody please help me..

You may want to have a look at the log file you're referring to. Does it say anything more specific about what's wrong?

Yes, a permission denied message when installing Wubi does not generally provide sufficient information. Please post the contents of the log file here (unless you can solve the problem yourself after looking at them).

These are some of the last entries of that log file.

08-03 19:36 DEBUG Distro: checking whether D:\ is a valid Ubuntu CD
08-03 19:36 DEBUG Distro: does not contain D:\casper\filesystem.squashfs
08-03 19:36 DEBUG Distro: checking whether E:\ is a valid Ubuntu CD
08-03 19:36 DEBUG Distro: does not contain E:\casper\filesystem.squashfs
08-03 19:36 DEBUG Distro: checking whether F:\ is a valid Ubuntu CD
08-03 19:36 DEBUG Distro: does not contain F:\casper\filesystem.squashfs
08-03 19:36 DEBUG Distro: checking whether G:\ is a valid Ubuntu CD
08-03 19:36 DEBUG Distro: does not contain G:\casper\filesystem.squashfs
08-03 19:36 DEBUG Distro: checking whether H:\ is a valid Ubuntu CD
08-03 19:36 DEBUG Distro: does not contain H:\casper\filesystem.squashfs
08-03 19:36 DEBUG CommonBackend: Searching for local ISO
08-03 19:36 DEBUG CommonBackend: Could not find any ISO or CD, downloading one now
08-03 19:36 DEBUG TaskList: New task get_metalink
08-03 19:36 DEBUG TaskList: ### Running get_metalink...
08-03 19:36 DEBUG downloader: downloading http://releases.ubuntu.com/11.04/ubuntu-11.04-desktop-amd64.metalink > D:\ubuntu\install
08-03 19:36 DEBUG downloader: Download start filename=D:\ubuntu\install\ubuntu-11.04-desktop-amd64.metalink, url=http://releases.ubuntu.com/11.04/ubuntu-11.04-desktop-amd64.metalink, basename=ubuntu-11.04-desktop-amd64.metalink, length=28363, text=None
08-03 19:36 DEBUG downloader: download finished (read 28363 bytes)
08-03 19:36 DEBUG downloader: downloading http://releases.ubuntu.com/11.04/MD5SUMS-metalink > D:\ubuntu\install
08-03 19:36 DEBUG downloader: Download start filename=D:\ubuntu\install\MD5SUMS-metalink, url=http://releases.ubuntu.com/11.04/MD5SUMS-metalink, basename=MD5SUMS-metalink, length=874, text=None
08-03 19:36 DEBUG downloader: download finished (read 874 bytes)
08-03 19:36 DEBUG downloader: downloading http://releases.ubuntu.com/11.04/MD5SUMS-metalink.gpg > D:\ubuntu\install
08-03 19:36 DEBUG downloader: Download start filename=D:\ubuntu\install\MD5SUMS-metalink.gpg, url=http://releases.ubuntu.com/11.04/MD5SUMS-metalink.gpg, basename=MD5SUMS-metalink.gpg, length=198, text=None
08-03 19:36 DEBUG downloader: download finished (read 198 bytes)
08-03 19:36 INFO saplog: Verified a signature from ID:'46181433FBB75451'.
08-03 19:36 INFO saplog: Checking block bindings..
08-03 19:36 INFO saplog: Key verified successfully.
08-03 19:36 DEBUG CommonBackend: metalink md5sums:
a941ca07230dd2a812b5abd3aa12f79f ubuntu-11.04-alternate-amd64.metalink
ec67d129102bbe53715c0a192c6e703b ubuntu-11.04-alternate-i386.metalink
350c4c0da5823625e5278cc869153e65 ubuntu-11.04-beta2-alternate-amd64.metalink
10eb1df66373fd00cd9be2acfc329bf3 ubuntu-11.04-beta2-alternate-i386.metalink
7b3ef94055a263d7f7f9b1ed6e3c99e2 ubuntu-11.04-beta2-desktop-amd64.metalink
84b89292c5ae40be000a5ee861da257e ubuntu-11.04-beta2-desktop-i386.metalink
d39c949129feb09da7e349882833f488 ubuntu-11.04-beta2-server-amd64.metalink
b7be563bfb89faaa4c7bd892a1a56aa1 ubuntu-11.04-beta2-server-i386.metalink
42aa52e7eaa441d0b00965d225f4e44f ubuntu-11.04-desktop-amd64.metalink
8376ce671b922bd1051f16ab2d44d06f ubuntu-11.04-desktop-i386.metalink
e23ed6d1164876804a7fb9dfb2f8aee1 ubuntu-11.04-server-amd64.metalink
b4b50d20f519fd0d8761e9d998630449 ubuntu-11.04-server-i386.metalink

08-03 19:36 DEBUG TaskList: ### Finished get_metalink
08-03 19:36 DEBUG TaskList: New task download
08-03 19:36 DEBUG TaskList: ### Running download...
08-03 19:36 DEBUG btdownloader: downloading http://releases.ubuntu.com/11.04/ubuntu-11.04-desktop-amd64.iso.torrent > D:\ubuntu\install\ubuntu-11.04-desktop-amd64.iso
08-03 19:36 ERROR TaskList: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 229, in listen_forever
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 221, in listen_forever
  File "\lib\bittorrent\Rerequester.py", line 96, in fail
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Problem connecting to tracker - <urlopen error (10061, 'Connection refused')>

Traceback (most recent call last):
  File "\lib\wubi\backends\common\tasklist.py", line 197, in __call__
  File "\lib\wubi\backends\common\btdownloader.py", line 79, in download
  File "\lib\bittorrent\download.py", line 303, in download
  File "\lib\bittorrent\RawServer.py", line 256, in listen_forever
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 229, in listen_forever
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 221, in listen_forever
  File "\lib\bittorrent\Rerequester.py", line 96, in fail
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Problem connecting to tracker - <urlopen error (10061, 'Connection refused')>

08-03 19:36 ERROR TaskList: Non fatal error Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 229, in listen_forever
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 221, in listen_forever
  File "\lib\bittorrent\Rerequester.py", line 96, in fail
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Problem connecting to tracker - <urlopen error (10061, 'Connection refused')>

 in task download
08-03 19:36 DEBUG TaskList: ### Finished download
08-03 19:36 ERROR TaskList: [Errno 13] Permission denied: u'D:\\ubuntu\\install\\ubuntu-11.04-desktop-amd64.iso'
Traceback (most recent call last):
  File "\lib\wubi\backends\common\tasklist.py", line 197, in __call__
  File "\lib\wubi\backends\common\backend.py", line 492, in get_iso
  File "\lib\wubi\backends\common\backend.py", line 347, in download_iso
OSError: [Errno 13] Permission denied: u'D:\\ubuntu\\install\\ubuntu-11.04-desktop-amd64.iso'
08-03 19:36 DEBUG TaskList: # Cancelling tasklist
08-03 19:36 DEBUG TaskList: # Finished tasklist
08-03 19:36 ERROR root: [Errno 13] Permission denied: u'D:\\ubuntu\\install\\ubuntu-11.04-desktop-amd64.iso'
Traceback (most recent call last):
  File "\lib\wubi\application.py", line 57, in run
  File "\lib\wubi\application.py", line 131, in select_task
  File "\lib\wubi\application.py", line 157, in run_installer
  File "\lib\wubi\backends\common\tasklist.py", line 197, in __call__
  File "\lib\wubi\backends\common\backend.py", line 492, in get_iso
  File "\lib\wubi\backends\common\backend.py", line 347, in download_iso
OSError: [Errno 13] Permission denied: u'D:\\ubuntu\\install\\ubuntu-11.04-desktop-amd64.iso'

It looks like wubi is trying to save the iso where it doesn't have write permissions. Try changing the target directory, or fix the permissions in D:\ubuntu\install.

You may also want to report a bug against wubi. It shouldn't crash in such a case, but instead it should pop a dialog telling the user about the permission problem and how to fix it

The problem is this:
DownloadError: Traceback (most recent call last):
  File "\lib\bittorrent\RawServer.py", line 221, in listen_forever
  File "\lib\bittorrent\Rerequester.py", line 96, in fail
  File "\lib\wubi\backends\common\btdownloader.py", line 70, in error_callback
DownloadError: Problem connecting to tracker - <urlopen error (10061, 'Connection refused')>

Wubi uses a bittorrent client to download the ISO. To do this you need to allow firewall access to the python runtime (pyrun.exe). If you did give permission to pyrun.exe then there may be an upstream firewall blocking the download.

To fix it, either give pyrun.exe access, or to get around the problem you can download and save wubi.exe, then download the ISO yourself through your browser and save in the same folder as wubi.exe before running. Download the ISO from http://www.ubuntu.com/download/ubuntu/download

Just want to add a couple of comments:
1. When you ran Wubi, you should have seen a popup prompt asking whether to allow pyrun.exe access to your firewall.
2. For more info on installing Wubi, see https://wiki.ubuntu.com/WubiGuide
3. In general you should post the entire log file (you should use http://pastebin.ubuntu.com as it can be very large). In this case it's okay because the failure is clear, but sometimes there are subtle problems and you need to see the whole thing. As you can see, if you read it from the bottom up, you can be easily mistaken about the problem - and the permission denied is really a misleading message.

Thanks for your comments guys,
I think bcbc is right because in my university, they have blocked torrents' downloads. Will try to get a dvd.

You don't need to obtain a physical DVD. Just download the Ubuntu .iso file at http://www.ubuntu.com/download/ubuntu/download. You do not need to be able to use bittorrent for this.

thanks

If you do end up having trouble downloading the .iso image, or figuring out what .iso image you need (you want the one for whatever version of Ubuntu you want your Wubi system to be), please let us know and we can provide more detailed help with that.