Open the My Music folder in My Documents.
Odds are your iTunes music is still there.
Copy all the music to some place safe, like an external usb drive or at least another folder on your desk top if it's too much to burn to disk.
Make sure you've got all your account information written down.
Now just de-authorize the computer and uninstall iTunes, while you're at it run this :
http://support.microsoft.com/default.aspx?kbid=290301
Now download a fresh copy of iTunes and save to disk and install it from there.
Recreate your account with the information you wrote down.
Re-authorize the computer and point iTunes to where you've got the music stashed and tell it to import the folder.
You'll probably have to re-do your play lists but you should be good to go.