Ignore me.
I thought you were asking for a torrent client to download and use on the machine :P
Best way would be to wipe and erase the contents of the hard drive using a partition manager.
Download your favourite flavour of Windows as an iso (image file) and burn it to a disk.
Once it is burnt, insert, reboot and install
If you are chasing a torrent client, utorrent gets my thumbs up!
Edit: Do the cases have a cd key stuck to them?
If they do what version of windows was the key for?
I have the oem installers for 2000/xp sp2/vista home premium floating around that I could create a copy for you.