Problem that annoyed me...

Recently I have got a client sent in his PC. His problem are,
1) Mouse will get hang or jam
2) PC will go restart without any notification

Things that he has attempt to do:
1) Change the power supply -- done.
2) Install a new 2G RAM -- done. Total now is 3G RAM running on WinXP.

What bothers me are,
1) His old 1GB RAM.
2) His CPU over heated.

However, I have been diagnosed his PC problem with the following steps.
1) unplug is old 1GB RAM. Leaving new 2GB RAM. --> problem still exist!
2) I am afraid his CPU temp hit the thermal throttling. So i remove his heat sink and fan. Clean it and put a new thermal heat sink compound.

Unfortunately, he called back after two days. "how come my PC still goes automatically restart while i am surfing internet half way?"

What a wonderful question! hmmm.. apparently I don know what the heck shall I do further. Any suggestion?

First thing may come up in my mind. Reformat his operating system see if it works. I am afraid there is something behind that triggers the system to restart.

Any mode of suggestion is welcome for the solution finding.