This starts to sound like a bad disk.... I once had to adjust my cd-rom drive settings in my BIOS to get windows to install right. You can check for something like that in your system.
About getting back to XP there must be a way using the "recovery console"
http://support.microsoft.com/kb/307654/
Goodluck
P.s. have to go now. Anybody feel free to step in. Later.....