XP Home BSOD

Hi
I wonder if anyone can help with an xp problem i have?
My specs:

Hewlet Packard Imedia 1517 desktop
Athlon 64 3400+ single core
XP Home SP3
3GB ram DDR 400hmz (2x1gb 2x512mb)
550W PSU
ATI HD4670
Audigy 2 soundcard

I began having problems a while back - while playing 'left 4 dead 2' the pc would sometimes make a high pitched sound in the headphones and the image on screen would lock up; alt-tab, windows key, task manager would not minimise screen so had to do reboot.
I began monitoring temps and psu was hitting 70c, so i cleaned the system (removed and cleaned ram sticks, fans etc) and re-applied thermal paste.
Temps improved. Then last night while playing left 4 dead 2, the pc froze up and made sound like many times before, and when i re-booted it wouldnt even load windows. I re-booted in safe mode and got black screen with scrolling lines saying 'multi(0)disk(0)rdisk(0)partition(2)\windows\system32\drivers........' following the word 'drivers' it lists many drivers from the system.
I re-booted agin just now and now get BSOD with message saying 'the bios in this system is not fully ACPT compliant' and it just hangs with that.

I've done some searching and it seems that removing ram sticks and loading one at a time may help so will try that now.

any suggestions are appreciated.
Many thanks, hope this is enough info.