UBCD won't boot on one computer, but does on another

[RobertG]

I know the copy of the UBCD I made is good because it boots up on my laptop, but not on my desktop pc.
Switched the bios so the system will boot from the cd drives before the HD, no luck. Tried it in both drives, nothing. After passing the drives, system goes to the HD and Windows xp Pro starts.
Made another copy at 2x speed, no luck.
Any suggestions?