P.C. won't boot

Hi guys,

I'm working with a friend on his XP home computer that will not boot. The mobo is a Asus P4P800 SE and when powered up it beeps once but there is no display. What we've tried so far:

Taking out all ram but one stick, then tried a different one.
We tried different mobo battery
Swapping video cards
Booting to a linux CD

Each time it still beeps once but still only a black screen.
Any ideas?

 

Sorry, I forgot to say that we tried that too.
When the problem started, his PC locked up and he got a 'Bad Checksum' message. The Asus recovery floppy with the latest bios didn't help and now it boots to only a black screen.
The fact that it beeps once when started but still nothing confuses me.

 

did you try to use a win xp disc ?

 

Yes, it won't boot with that or a Linux bootable ISO, just a black screen.