I was encoding a DVD the other night and i left it alone and when i returned the CPU fan was working overtime making a right racket and was greeted with black screen, since then have been getting random crashes and will only restart after it has cooled down.

Any ideas how i can check whether it is knackered or not? wondered if there was a program which could check.

Have you looked inside the case? is the cpu fan covered in dust? have you tried another fan to see if it still the same? you may be lucky and not done to much damage