so after it posts, nothing loads....
sounds like you lost your operating system or your harddrive. your best bet is to format the drive and reinstall your OS. if you can't format the drive or access it. the hdd is dead.

if you have a s.m.a.r.t.s compatable drive, turn that on in your bios. that will report hard drive errors to you before you drive goes... (or is supposed to, doesn't work right all the time)

Before formatting, you might try booting into rescue mode from the XP installation CD. Let it try to locate and repair your installation.

Did anything bad or unusual happen to the computer just prior to it refusing to boot?

Definitely... I'd grab some type of disk checking utility, too. Your hard drive might be trying to tell you it's not happy...

Have you tried booting into Safe Mode?

LittleMan