PC reboots regularly

Check out your event logs. They will tell you if something errored out before it rebooted. If you see nothing, then you can start looking into hardware failure. There are some good boot CD utilities to check for hardware/memory issues.

The RAM is most likely the cause on your problem. It's cheap, so maybe you can just swap in another piece of RAM just to see if it helps.

"Temp sensors may not always be accurate. If you suspect that heat is the problem it's easy to diagnose.

Open the case.
Clean out dust.
Point a table fan a the inside of the box.
If the problem goes away you have a heat issue.

Place the fan far enough away (several feet) that any bad EM/RF it emits will not affect the PC but it still keeps the inside of the PC cool.

Most heat issues are due to dust or other airflow blockage or a failed fan."

I would also check into your hard disks. Run SpinRite (I should get a referal account with Steve) and see if that fixes your problems. http://www.grc.com/cs/prepurch.htm The program is $89, but if it doesn't work for you Steve will give your money back.