Try to boot up a different machine with the DVD, it can be an old not vista-compatible one. Just to see if it boots.
If it does, it is very likely that the downloaded iso is corrupt or you use a wrong version (x64 on 32bit hardware ) .
Search in the forums about the iso-checksums ( you find on the download page of RC1 ) and how to verify these with approbiate tools.
If they mismatch, then you must downlaod and burn again
SBJ