If the computer is still up and running, you can extract your Windows key from your system with a program made for those things. Then back up anything you want to save. After that you simply go fetch your Windows version from a safe download site and put it as an ISO file so you have a bootable CD or thumb drive. And totally start over with a new Windows copy and your valid Windows key.

That would create an original Windows copy, not sure if it is what you started with. Some computers come with Windows tweaked for that specific brand and/or with extra software, this way you won't have that. And that might not be ideal if your version included something extra you actually paid for. Some of these things also come with extractable keys though.