Microsoft's suggestion for a fix:
To reset your taskbar to the default position, restart your computer in Safe mode. To start your computer in Safe mode, restart your computer, press and hold down the CTRL key after your computer completes the Power On Self Test (POST), and then choose Safe Mode from the Startup menu. The taskbar will reset to the default position after you reboot your computer.
NOTE: If you press and hold down the CTRL key when your computer starts, you may receive a keyboard error message. If this occurs, you may safely ignore the keyboard error message.