I would have said your boot.ini file, (or Vista's equivalent if Vista is different), had been corrupted by the XP install on the other hard drive, but since the system stalls on POST and does not even display your hard drive, I would first check your BIOS settings.
Has the hard drive been removed from the boot sequence list?
I recently took a hard drive and put it in my computer to install windows xp on it for my neighbor. I left one of my harddrives in my computer which runs windows vista. Now after removing neighbors hard drive and putting mine back in to original state my computer will not go past post screen. It says that it is entering setup but just stays on post screen. My computer is a Sony it is fairly new and one of the better computers as far as speed, upgrades, etc. I've tried everything, checked all connections but to no avail. What else should I try. Processor is a pentian 4 3.8 ghz. vgc rb38g is one of the model numbers, and the other one is pcv-c21L. It has an intel desktop board d915gag/d915psy is number. I hope that this is enough info. Please help! Don't know what else to try. Thank you!!!