If you know when this problem started, you could do a System Restore to the point immediately prior and see if this cures it. If so you can then clear System Restore by turning it off, rebooting and then turn it on again.
However, if you have installed new software it could be caused by that. To find out uninstall it and again reboot to see what happens.
The other possibility is that you have changed a setting somewhere or corrupted the Registry; again a System Restore should resolve this.