clean computer
Use this to scan for what's bad, it should clean it up if it's not too messed up:
http://www.malwarebytes.org/
Run this to help prevent spyware, it doesn't clean anything but helps stop you from getting it:
http://www.javacoolsoftware.com/spywareblaster.html
If you don't have up to date anti-virus program, use Microsoft's free one that is really good:
http://www.microsoft.com/security_essentials/
If you still get random pop-ups, spyware looking stuff, or your homepage keeps changing, you should backup all your data files (pictures, video, docs, xls, etc) and do a system restore, then do all the Windows Updates and reinstall your applications.