Can anyone advise me on this please?

I have a Toshiba Satellite notebook - XP home edition. Since installing SP2, I notice that after a few minutes since logging on, the notebook for no reason, it completely shuts down. Tried a system restore but still doing it.

The notebook spec that I recall is as follows: 1.7ghz processor - pent III, 256mb ram, 20gb hd.

It's rarely used so I don't know what is causing this problem.

I have checked for spyware & viruses, & nothing has been detected. I have even recently re-installed Windows XP but after about 15 mins it happens again.

Is there anything I can do / check to prevent this from happening?

Thanks in advance!