Since the only stupid questions are the ones that are never asked..
..Where did you download the ISO from? If you got it from a torrent/p2p then its possible it could be altered etc. If you didnt already do so, try getting it from the official site
http://www.ubuntu.com/getubuntu/download
As dragonslayer suggested, try burning it at a slower speed and use a recognised burning app such as nero or roxio (if u download the iso onto windows)
Hope this helps
Regards