I would start by booting to Safe Mode and doing a Virus scan.
If it is not a virus then try a System Restore to an older restore point.
If you do not have and restore points then you could try a System Restore DVD if you ever created one.
Or if you have a Windows 7 Installation DVD you could try booting to that DVD and reinstalling over the existing install.
Safe mode info from HowToGeek:
How to Use Safe Mode to Fix Your Windows PC (and When You Should) (howtogeek.com)