A system restore is the best idea. Simply go to Windows system restore and restore your settings back to a point in time when you know this was working fine. You can always roll back a system-restore in Windows and it doesn't hurt anything you've done on the computer since the restore-point you decide to use. It's very strange to have this just all of the sudden happen and at least with a system restore you can figure out if it's the O/S or not.
If a restore point fixes the problem then you have not only fixed the problem, but also narrowed it down to a setting that got changed, or some software incompatibility, or even possible malware infection.
Oh and I think linney wanted to know if the problem still exists when you boot to safe mode - pretty self-explanitory.
Good luck,