The taskbar appears on the wrong monitor after you change the primary monitor in Windows XP

To resolve this problem, follow these steps:

1. Click Start, click Run, type desk.cpl in the Open box, and then click OK.

2. Click the Settings tab, and then click the monitor that you want to use as the primary monitor.

3. Click to select the Use this device as the primary monitor check box if it is not already selected.

Note: The Use this device as the primary monitor check box is unavailable if you click the monitor that is currently set as your
primary monitor.

4. Click the monitor that you want to use as the secondary monitor.

5. Click to clear the Extend my Windows desktop onto this monitor check box, and then click Apply.

6. Click to select the Extend my Windows desktop onto this monitor check box, and then click Apply.

From: http://support.microsoft.com/kb/905776