First, take time to back up your personal and important info. If you don't have a method of doing this, find a local computer shop or chain store like CC or BBY and have them back your stuff up. Remember it's not cheap, but you know they will get it for you (if you tell them where to look).
Hopefully, you purchased the computer from a retailer. If so, your computer will have a method of returning it to the state it came originally. Old computers came with a kit of CDs, newer units have a special function like hitting F10 on startup, a DVD or a hidded option in the Control Panel.
Look in the information that came with your computer for the word Restore.
If you don't have these options, call the manufacturer with the Model and Serial numbers handy to order the restore kit.
Good luck, and try Avast or Kaspersky. They're some of the better antivirus / antimalware brands out there.