Help My Scan PC just shuts down?

The PC 4 years old now but is perfectly satisfactory for my current needs, however it has developed an annoying habit of every day or so it just shuts down, like someone has switched it off at the mains. It only does this while it is doing something processor intensive (ish)...it will sit doing nothing for days/weeks on end and not shut down, but running say a registry clean up program or running a few programs at once and suddenly pop...it switches off.
I have replaced the power supply to no avail, any ideas??

check the cpu temp? the heatsink may need a bit of cleaning.

How do I check the temperature? what sort of range should it be in??

Defrag hard rive.(when did you do that last?) , take unnecessary things which run at startup . Run spy ware , and then run tuneup utilities 2006.

I do reckon you have a virus and also your hardrive is in a mess.

Agree it sounds temperature related, check cpu temps and make sure your cpu fan / cooler are functioning

Use speedfan to check your temps http://www.almico.com/speedfan.php

the actualy max varies by processor, but it shouldn't generally be above about 60. Some P4's can handle higher.