Did the whole task bar move with it?

Can you right-click the task bar, choose Properties, then on the Taskbar tab change the "Taskbar location on screen" back to "Bottom"?

You should also be able to frag the start bar to its position at the bottom.
Once you have managed to get the task bar at that position best is to lock the task bar via the right-click context menu to prevent this from happening.

Also see

• http://support.microsoft.com/kb/253679

AND

• http://windows.microsoft.com/en-US/windows-vista/Unlock-and-move-the-taskbar

This question is about Windows, not Firefox, so I'm closing the thread. See Mozilla Support rules and guidelines : Posts in the Firefox support forum must be either questions about the use of Firefox or answers to those questions. Users who post about things other than Firefox support will be directed to an alternative discussion place (ex: mozillaZine) if appropriate.

If you are still having problems moving the Windows Start button back to the bottom left-hand corner, your best bet would be to visit a Windows forum such as answers.microsoft.com or windowsbbs.com and ask for help.