A bit more info after some more 'dabbling' -
Everytime I put a new piece of software on it does the same thing. If I unplug the PC, turn on and reboot and everything's fine, until I put a new program on. It's not the HDD as I've tried it in my PC without a hitch, and I don't think it's the software as I've tried 2 different copies of XP Pro and a copy of XP Home with the same results.
Possibly the motherboard or processor?