Shalazar

ok here is the situation. I bought a second hand comp reasonably good : 2.6 Ghz, 120 GB, 512 Mo RAM, 256 Mo Graphic card and upgraded it myself to a nice 3.2 Ghz (overclocking: I paid attention to have a motherboard that resists it very wel), 4GB RAM . However this guy had an illegal version of windows xp pro on his comp. Since I need a reliable system for my work: I wanted replace it with the original xp home. But even though the key for windows xp home is still on the exterior: I have no idea how to proceed. I dont want to use a download: cause if ever I get in trouble with viruses I wont be able to re-install, and I havent managed to get ahold of a way that would allow me to get the CDs. (the guy I bought the comp from threw them away) So, any ideas?