Right click on my computer and click properties, scroll down all the way and wait a while and see if you get a product ID code. If the key is from a computer that came with windows like Dell, HP, etc. then call them, you can also call Microsoft.
You havent given us any information and what I have said doesnt make much sense because if you did buy it from a company it would come pre-installed...even if you did build it yourself you would have entered it during installing.
If you pirated this you truly are a handicap, the software could have edited files or viruses somewhere deep in windows like the registry or something.