Last edited by Andrey Bushman; July 6th, 2012 at 09:18 PM. Reason: Fixed some grammatic errors
This isn't really the best place to mention problems with the ubuntu website.
You could try downloading the image from a different mirror.
You should be able to find the MD5 checksum for the image, but I can't seem to find it on the ubuntu website.
The MD5 check as trundlenut mentioned.
Also, what is your installation media (hard drive, USB stick, DVD drive)? You may need to change that.
Also if you download the ISO as a torrent you can be guaranteed that it's OK.
Torrent downloads check the hash of the file as it is downloading as part of the bittorrent specification.