First of all, right click the taskbar to make sure that "Lock the Taskbar" isn't ticked (it probably isn't by the sound of things), then left click and drag the taskbar to its normal position at the bottom of the screen. When you've got it in the position that you want and it's the correct size, right click on it again and tick "Lock the Taskbar". This will stop you accidentally moving it again.
Failing that, you might want to try System Restore assuming you've got a suitable restore point set. Click "Start->Accessories->System Restore and follow the on-screen instructions. This should restore your computer to the state it was in earlier when (presumably) you didn't have the problem.