Keyfinder Thing v3.1.6 works for nearly any Windows operating systems except Windows 7 and Windows NT. You need the original product key that came with your Windows purchase before you can reinstall Windows.
For a quick overview of what Keyfinder Thing can do, see my complete review of Keyfinder Thing v3.1.6.
Keyfinder Thing is a free software program that finds product keys and serial numbers so the first thing you need to do is visit the Keyfinder Thing website, hosted by Download.com, so you can download the program.
Keyfinder Thing is a completely free program and you should not be charged anything to download or use it.
Note: The detailed instructions I've put together here walk you through the entire process of using Keyfinder Thing to locate lost product keys and serial numbers so feel free to take a look at the entire tutorial before you get started.