In dire need of help !

I'm having a problem connecting to the internet, that is, whenever i try to connect to my wireless hub via a WiFi connection (i can freely open internet explorer, but it does nothing as the wireless network isn't up and running).

Each time do, my computer immediately shuts down. This only started a few days ago, totally out of the blue. I have neither installed new software or hardware.

There is another instance when the computer shuts itself down immediately, and that is when i type cmd in the dos prompt, it hasn't happened while doing anything else as yet.

I don't think it's an overheating issue, and the computer will run indefinitely provided that I don't do either of the above, and the fan is clear.

When the problem first occurred I formatted my hard drive and reinstalled windows (XP) and all the drivers, but that hasn't made any difference.

A malware specialist has taken a look at my virus/malware checker logs and told me that there is nothing of that sort on my system.

So I'm kind of at a loss at the moment and I'm hoping someone can help please !

Thanks in advance