When you enable multiple monitor on Windows XP, which I did by installing an ATI Radeon card in addition to my existing Nvidia card I found that after changing some of the settings (like which is the primary display), the Start Menu and Task Bar appears on the secondard display. What seemed to happen was the Start Menu etc. moved to one of the display's managed by the ATI card and then I set the primary display back to the Nvidia card it became the primary display (programs and dialog boxes opened on that display) but the Task Bar did not move back.
Eventually, after much reconfiguration and reseting/rebooting I decided to see if I could drag the Task Bar across. Now this is not supposed to work (maybe it has been added in SP2 for XP). So I unlocked the Task Bar and dragged it first to the side of the current screen (the displays are lined up left to right) and then across to the other display - you cannot drag it straight across the bottom of the displays.
This worked a treat, so I relocked the Task Bar and all works now.