I downloaded Ubuntu 15.4. I'm getting errors while creating an USB.

Did you MD5 test the ISO you downloaded?

There was no option to check MD5 as the download happens through http link:

I'm trying to upgrade to Ubuntu 15.04 from Ubuntu 14.04. The USB writing tool; Startup Disk Creator, failed and went into unending loop and the tool window does not close. After repeated attempts, with USB and DVD writing, I got the normal exit from the Startup DC tool displaying that the USB is "good to upgrade in other computers."

At the early attempts, I got a message as follows "could not find Kernel Image:/isolinux/isolinux.cfg - - boot: ", at the boot time. Unable to understand the error source, I tried different USB sticks and at last a DVD in all the cases, the writing failed at the early attempts and finally normal exit occured saying as above.

I commenced the up-gradation process after booting with this USB and DVD. But to my frustration, the process went in to an unending loop at the stage of "saving packages.." and the up-gradation won't happen. I had to shut-down the system as there was a no normal exit from process window.

I tried writing the through Rufus (star-up disc creation tool) in Win 8.1. Rufus tool reported that "This image uses Syslinux 6.03/20150318 but this application only includes the installation files for Syslinux 6.03/2014-10-06. As new versions of Syslinux are not compatible with one another, and it wouldn't be possible for Rufus to include them all, two additional files must be downloaded from the internet ('ldlinux.sys' and 'ldlinux.bss'):

Rufus downloaded the above files and USB for upgrade/installtion is made. While attempting upgradation through this USB sticks, I get an error messge, it is a very short display stating that " ..... failed" cleared by installtion GUI screen.

Yes, MD5 testing is a manual process so it is totally an option. That's like saying that moving a file to a new location once downloaded wasn't an option.

You have not tested the data you have downloaded so have no way of knowing if it is complete or consistent. TCP does a good job of rerequesting bad packets but it is not infallible. Using torrents helps as the torrent protocol adds extra error checks.

https://help.ubuntu.com/community/HowToMD5SUM

Also try the PenDriveLinux 1-2-3 installer to make the USB install media.

I suggest using multisystem instead:

http://www.pendrivelinux.com/multiboot-create-a-multiboot-usb-from-linux/