i'm trying to find out what is up with my pc

it crashes ALL the time, sometimes after 2 mins, spometimes after 3 hours but it alwasy happens.

so far:

i did a ram check...and thats fine
all the cpu temps etc have been fine
i replaced the hd, but it kept crashing during the install, so i have a new HD to use when i have fixed my pc.

as far as i know the psu is fine so i have put it down to either the cpu or the motherboard. which is it nore likely to be...as i would like to replace one or the other not both. its only and AMD 1.4 and only used for itunes and internet surfing so just need a cheap fix

any guidance appreciated

rory