Grab your Windows disc, backup all important (personal, not system) files and then reinstall.
Save games, photos, music... Get it all on some backup disk.
What? No Windows disc? Doesn't matter. If you've got a valid license, then download it. It's legal as long as you get the correct channel (probably retail or OEM) and you have a valid serial (if there's a sticker on the back of your machine, you've got an OEM license, if you have a manual or a box, you have a retail license).
You might want to give your backups a final hosing down with an Antivirus that isn't Norton or Avast (AVG Free is your best friend here).
Then reinstall Windows. If you paid for Norton, demand a refund and then don't use it ever again. You really should be using AVG Free as your antivirus, and paid if you can really afford it, even if it's just for losing the nags.
To be totally safe, you should completely format the disk. Do not do an in-place upgrade (which in Vista and 7 is to have your current install moved to Windows.old).