Is it possible to expand my existing 25GB Ubuntu Partition ??

You can do a few things:

1: run: sudo apt-get clean

This will delete the debs used in updates.

2. Remove old kernels

if you run:
uname -a

You will see your CURRENT kernel version. If you then run:
dpkg -l | grep linux-image-2

You will see the INSTALLED kernels. If you remove the kernels you do not need but do NOT remove the CURRENT kernel, you will gain ~120Mb per kernel.

3. Install lighter Office productivity.

OpenOffice is an absolute beast weighing in at nearly 400Mb. If you remove this and install Abiword and / or gnumeric you can save 200Mb easily. You can also use googledocs etc and require no office suite installed AT ALL but will require web access to word process etc.

4. Remove ubuntu-docs

Again, this is a whopping 250Mb and is a lot of stuff you'll never read if you have a web connection most times. It's pretty useless and can save you a lot of space by removing it.

5. Run bleachbit

Run it as both root, then as user. be VERY careful of settings or it will clean browser settings etc, also avoid settings which say they will take a long time. Have as many apps closed as you can as this will release files from app-lock

All these will help you reduce used space. My root partition is 3Gb used and I have 75Gb for /home on its seperate partition.

If actionpartnership's solution doesn't provide you enough space, you can resize NTFS partition to make place to Ubuntu partition.
Best practice is to do a backup of both partition, but until now I had any problem with partition management.
Easier is to work from Ubuntu CD, which will:
   provide you Gparted software already installed
   avoid to mount Ubuntu partition.

Boot on Ubuntu CD with live session (option "Try but don't install").
System->Administration->Gparted
Gparted will display aside both NTFS and ext4 partition.
You have to:
   unmount NTFS and/or Ubuntu partition if one of them is mounted
   move/resize NTFS first, choose to reduce NTFS by freeing space at END of partition
   move/resize ext4 partition, by extended it at freed NTFS space
   execute commands by clicking on green arrow
Rebooting iIndows and after Ubuntu to check all is right.