Master Boot Record corrupted?

[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)

[Teachme]

If you think it is due to MBR corrupt then boot into xp repair console using XP boot CD and type fixmbr in it.
Try this and check if it helps.

[adalgisa]

I don't have an XP installation CD because it's a Compaq. Is an XP boot CD different? I tried booting using UBCD but it didn't work.

[glc]

If you recently installed a DVD writer, I suspect you may have the hard drive and/or the optical misjumpered. You most likely have cable select type cables, which have to be installed with the correct end on the motherboard, the master installed on the other end, the slave in the middle, and both drives set to CS (CABLE SELECT), not master or slave.

[adalgisa]

The jumper is set to cable select. This was a replacement drive- I just pulled the cable off the dead one and put it in the new. Didn't touch the mobo-

[glc]

Was the jumper on the old drive set to cable select? What happens if you disconnect the optical?

[adalgisa]

Yes, it was set to CS on the old drive. I'll try disconnecting the optical drive after I back up everything-- I'm afraid if I turn off the computer to open it up I won't be able to start windows again-- and then I'm stuck until Monday when I have access to another computer.

[adalgisa]

Windows is starting normally again...no, I have no idea why.
The only thing I did was change to a gui boot in msconfig, instead of nongui boot, so I could watch the little windows logon and zippy blue thing.
Well, at least it made me do a backup.