Buy another OS? If this machine is a prebuilt tower, if you look on it, usually on the top or on the side, there should be an OEM key that was applied by the manufacturer of your pc. For example, if it's for Windows 7 Home Premium, if you have any friends that have a standard vanilla Windows 7 Home Premium disc, you can use that disc to reinstall Windows 7. Just when you reinstall, you install using the cd key affixed to your pc. You will have to go through trouble of finding drivers and what not, but the key attached to your tower should activate and be legal for you to use. If you take the pc to a repair shop, expect to pay a diagnostic of some type and then 100 bucks or a little over for them to install the OS and drivers for you.