On Vista try this:
1. Click the Windows 'Start' button; click 'Settings'; click 'Control Panel'; and choose (double-click) 'Personalization.'
2. Choose 'Display Settings.'
3. Click the 'Identify Monitors' button to see which one Windows calls 1 and which it calls 2.
4. From the drop-down menu below the monitor thumbnails, choose the one you want as your 'Main' monitor.
5. While this is selected, click (check) the box labeled 'This is my main monitor.'
6. Then choose the other monitor.
7. While this one is selected, click (check) the box labeled 'Extend the desktop onto this monitor.'
8. While each monitor is selected, you should probably set the resolution to 'Highest 32-bit' on each.
9. Then click 'OK' and close the various windows that are left open.
If this doesn't work, try screaming, and then tell us how far you got with it.
Good luck!
Eric