My computer reboots (restarts) after about 3 minutes when Windows opens and the desktop is shown, pl

I'm using Windows XP Professional Service Pack 2. The problem is that after about 3-4 minutes when windows opens and I run any program (or even wait a little bit) the computer suddenly slows down and restarts. Please provide me with any steps or software that I can use to restore my computer.

Please keep in mind that:

1. I've tried fixing errors in the registry with powerful programs, I made a full system scan (the antivirus was last updated 2 weeks ago.) I've also tried using system restore but all this didn't solve my prblem.

2. Safe mode works fine.

3. I can't connect to the internet in safe mode so that's why I didn't update my antivirus.

4. Not able to connect to the internet doesn't mean I can't do any thing online or download software, as I am using right now a laptop (I wrote this question using it).

Thank you very much for your help, I will strongly appreciate it.

Sorry, because the question was long, I know it was boring to read all this

My computer reboots (restarts) after about 3 minutes when Windows opens and the desktop is shown, please help?husqvarna

when you boot your machine and press F8 choose 'safemode with networking' update your virus def's and update from microsoft too.

My computer reboots (restarts) after about 3 minutes when Windows opens and the desktop is shown, please help?microsoft office