A few days ago a customer dropped off a desktop computer that would not boot. It was a Compaq model about three years old and, according to the stickers, it had Windows XP Home Edition installed. I booted the machine and was met by a message saying that a .dll was missing. I rebooted the machine using a Linux livecd. Examination of the Windows partition showed it to be completely empty. Also gone were all installed programs and all personal data. Apparently this person was the victim of some serious Windows malware.
The machine had a recoverery partition so I booted into that. Recovery failed after about 90% complete. The partition was damaged. My final option was to download an XP Home .iso from a bittorrent site. This installed nicely. This is not necessarily illegal as the Windows install now requires activation. Of course. my customer had no knowledge of product keys or activation. I told her that the computer would only work for 30 days without activation and wished her good luck. I also gave her a copy of Linux Mint to install when Windows stopped working.
There are a few Windows utilities that will reveal the OEM install product key. Here is a link to one. If you are still using a legal copy of Windows, I strongly reccomend that you retrieve your product key. It may save you some grief in the future.