Not possible. Here is a good prebuilt used system; http://www.ebay.co.uk/itm/Dell-Precision-390-Desktop-PC-Core-2-Duo-E6300-2x1-86GHZ-2GB-250GB-ATI-GFX-/121154892654?pt=UK_Computing_DesktopPCs&hash=item1c3564f36e
You would have to find a windows XP install disc to install an OS on to the computer. Also, there is no guarantee the computer will work, as the seller description states. Considering the price of an OS though, if you need an OS, this would be the way to go.
Your best option is probably trying to find used/ old parts on ebay/ amazon and then assembling them. You won't get a very powerful/ good computer for 100 pounds though.
Not possible. Here is a good prebuilt used system; http://www.ebay.co.uk/itm/Dell-Precision-390-Desktop-PC-Core-2-Duo-E6300-2x1-86GHZ-2GB-250GB-ATI-GFX-/121154892654?pt=UK_Computing_DesktopPCs&hash=item1c3564f36e
You would have to find a windows XP install disc to install an OS on to the computer. Also, there is no guarantee the computer will work, as the seller description states. Considering the price of an OS though, if you need an OS, this would be the way to go.