Did you verify the checksum before burning the ISO?
If so did the md5 or sha1 sums match?
How did you record the ISO?
What kind of hardware are you using and what version of ubuntu did you download?
If you downloaded the AMD64/x86_64 version and are trying to run it on i386 hardware it is not going to work.
Also did you verify the media after recording it?
Please be as specific as possible.