If you can't find the sticker anywhere on the tower (or laptop), you need to boot into Windows & run a program such as Magic Jelly Bean keyfinder. If you can't boot into Windows to recover the key, try one of these solutions:
The information on Computing.Net is the opinions of its users. Such
opinions may not be accurate and they are to be used at your own risk.
Computing.Net cannot verify the validity of the statements made on this
site. Computing.Net and Purch hereby disclaim all responsibility
and liability for the content of Computing.Net and its accuracy.