Ok, i have some problems with my Pc.

A few days ago i did a Defrag and during this the computer crashed. After i next rebooted the computer would always just randomly restart at any time or go to the blue screen of death at any time. As it is not a heat or power supply problem i decided to re-install windows. Just after the installation finished i got another blue screen of death.

Please any help would be good. I have no idea what to do...

Could you post the errors you get for the BSOD? Also, what OS is this?

Well, in fact i have more problems now.... After Re-installing windows Xp the computer no longer goes to the blue screen but just restarts. I have checked if it is the Automatically restart when an error occurs and its not. I have found it only restarts when i load up a game. Does this indicate the power supply? And how do you know if a power supply has 'had it'.

When you "re-installed" it, did you just install XP and overwrite what was there, or did you clean install in which XP reformats the hard drive?

If you clean install XP you will lose all of your data and programs on the hard drive, which is a drag to replace everything, but formatting the drive will find any bad sectors on the hard drive and not allow programs to be placed on a bad sector. Programs in bad sectors are corrupted and will cause problems.

As to your specific question about the power supply, an older computer is a likely candidate for a power supply failure as is one that gets spiked by a higher voltage or if your machine is not well ventilated, power supplies will start acting up.

Given what has happened to your machine and what you observed happening when it happened and what it does now with games, in order of preference I would suspect 1) the hard drive, 2) possible RAM damage, 3) video card, 4) audio card, 5) a motherboard, and then only power supply.