PC shuts down...aargh!

Hi,

Problem started off as PC shutting down randomly. Cleaned dust from inside case, around processor cooling fan and PSU. Now it shuts down during XP boot up (still on the black screen). When it shuts down the processor cooling fan stops, the HDD LED remains brightly lit and the PSU fan remains running. I can access the BIOS set up and it will sit happily on this without shutting down. Looks like it shuts down when the precessor starts thinking. I have not added any harware or anything. I do have protection: anti-virus, malware, spyware, etc and keep these up to date.

I was thinking that I may need to change the processor cooling fan (inc replace the heatsink compound) or the PSU?

Any help and any comments welcome.

 

try starting the pc up with the cover off of the side, try pointing a fan
on the cpu/motherboard

 

Footloose,

tx for the reply. tried cover off and fan on motherboard/CPU with no luck - still shuts down.