My PC reboots itself after 30 - 40 minutes. I can be doing a task then the compter freezes up, the mouse doesn't work, you can still see the screen. Then it reboots itself, during the boot process sometimes it gets through the memeory test or locks up at that point. Then other times it gets to the IDE dectection then goes no further. After 40 minutes or so I can start it up again, but I know I only have another 40 or so minutes before it locks up and reboots itself again... Any ideas out there?

Sounds like heating problem, check your fans Including CPU Fan

Hi. Aas it starts after a while it sounds like a heat problem. Make sure the fan is working and not gunged up with fluff.

Hello, does it reboot suddenly after it freezes or does it show and error message and a countdown?

First as guys said that Heat may be the Problem if not then
First go and check your BISO Setting for CPU Temp and if u are using P4 I recommend that you should Test your RAM by any RAM testing software and by Changing it to find out the real Trouble...