Well, what does it do? Does it have a separate cord for power? Does the computer make any noise or open anything when you plug it in? Does the drive make any noise when it has power, as the drive inside should spin and make some noise? Does it work on any other computers? Did you try anything else other than plug it in?
You say it works on Vista OS. Does that mean, it works on a DIFFERENT MACHINE that runs Vista? And, do you mean that the SAME LG Unit works under Vista? When it "works", does that mean it already has files on it you can read and write? That may mean the problem is in hardware - like power supply or USB ports - not the OS. On the XP machine where the drive does not work, are the USB ports Enabled? Does any other USB device work on that machine? Are you plugging the drive into a USB port right on the computer, or through a USB Hub unit?