Need HELP!

I have WIN XP installed. The (problem 1) is that when the computer left alone(nobody works at it) it restarts spontaneously after some period of time( an hour or two).
Interestingly, that doesn't happen when you work at it in WORD or I-EXPLORER for example!
Also,(problem 2) if CPU is loaded ( 3D screensaver, Hot CPU tester or ANY other application which
needs much resourses) it restarts, but now in just a minute or two after starting such an application.
Now, after i unchecked the AUTOMATICALLY REBOOT AT SYSTEM FAILURE, problem1 gone, but problem 2 remained.
For now, I don't even understand wether this software or hardware behind it!
I'm wery thankfull to everyone for sound advice!

I would suggest running Event Viewer (in order to access the error logs, and check for any critical errors)...Start | Run | Type eventvwr.msc and hit OK | Highlight "Application" and "System" independently in the LH pane | Right click the error message(s) (critical errors are in Red), and select "Properties" | Write down a couple of the error messages exactly as they appear (under the "Description" field), and post them in this thread.

Welcome to the forums.
You could be experiencing a heat buildup. Is your rig overclocked?

Sounds like a heat problem to me also.

A heat problem on idle? This really sounds like a memory problem , maybe a bad ram chip or a program that slowly eats away at the memory til none is left and it will reboot because it need to free up the resources. This could be ram or a program eating the memory up. Maybe someone can help on figuring out which it may be. I would suggest running memtest to check the ram out. Trying to figure out if its a program would be more difficult to diagnose.

Tread

Thank you everybody for assistance!

Some notice:

I started 3Dscreensaver and in a few minutes the computer restarted. I looked in the event viewer,
but there had been no error report, neither in application nor in system log !!!

What should it be then, hardware ?

I t seems to be memory to me. Have you tried the memtest yet?


Tread

I havn't. What program should I use for it,
and how it will indicate that the memory is bad ?

Thank you!

Its easy enough to build up heat on idle if you don't have adequate case ventilation. Heat keeps amasssing instead of being vacated from the case. Also, CPU-intensive programs will add greatly to the heat level.