Windows XP Install not starting using Boot Camp

I've got a custom XP disc slipstreamed with SP3, as well as a legitimate WMC XP SP2 disc, neither of which will start installing after restart when I select "start windows installation" from Boot Camp Assistant. I know that the discs aren't bad because I can use them on a PC and on VMWare Fusion.
When I restart, the CD whirs around or a while then spins down and I'm stuck at the blank white screen-- the XP installer nor OS X loads. So then I have to power button off and restart, with the only way of getting OS X to load is by holding the eject button (whether there's a CD in there or not).
I've done a ram reset, checked the HDD's, even restored to time machine backup and tried again with same result.... Even AppleCare on the phone didn't know what to do except take it to a Genius Bar.

Anyone dare to venture a guess?

Thanks!

PS: iMac 24", 2.8, 320GB, 2G, OS X 10.5.5