[adalgisa]

This afternoon the computer would not boot up; it got to the screen that asked if you want to boot into XP home or Recovery console, and then...blank screen. Multiple attempts were unsuccessful, but twice (in 4 hours) it has booted into XP, I don't know why.
What is causing this?

I tested the HD using seagate's diagnostic, and it was ok. (HD was replaced a few months ago)

The power supply is an FSP 350watt- it's a couple of years old. Seems fine--

Malware scan at ewido.net didn't turn up anything. HiJackThis didn't show any funny business.

I was unable to start the recovery console from the menu. Blank screen again.

When I tried booting -- I think it's called safe logging, or something -- where it shows everything in the boot sequence, it hung at a file called "agp440.sys"-- which I figured was related to video. But it wouldn't boot in VGA mode either. The mobo has onboard video, but I put in a GEForce 5500 into the AGP slot a couple of years ago, and it's worked fine.

Could it be a corrupt MBR? or NTLDR? I tried making a bootable XP floppy with a copy of NTLDR, boot.ini and NTDETECT.COM. No boot.

I have a UBCD but I don't know how to use the utilities on it. I didn't feel like experimenting, since I hadn't backed up files since July!

Since it's a Compaq, I don't have the WinXP installation cd. I have a couple of system restore DVDs that will do drastic things.

My S.O., who is the main user of the system, insists nothing unusual happened. I installed a DVD writer a couple of weeks ago, but it seems to be working fine.

I'd appreciate hearing from you experts-

System:
Compaq Presario from 2004: XP Home, Seagate ATA drive (a few months old), GEForce FX5500, Lite-On DVD writer. Asus mobo A7V8X-LA (which, being something created for HP, I can't find on Asus' site) Via chipset-. AMD Athlon 3000+ cpu. 1 gig RAM (crucial & infineon) (The computer that made me vow to never buy from Dell/HP/etc again)