If the reboot doesn't help, then your user profiile may be hosed. I've seen Windows 2000 do this when using some other progs and the only solution was to make a new user profile. It got to the point where you couldn't even open the My Computer icon.

If you only have one user, you may have to start in safe mode (Hit F8 just before XP logo fades in during startup), choose administrator and make a new user.

Cheers!