Try this, it works on my desktop XP:
1. Right-click My Computer and select PROPERTIES | HARDWARE | DEVICE MANAGER
Are there any yellow exclamation marks shown? If so, you'll need to uninstall these drivers and reboot
2. Plug in the monitor
3. Right-click My Computer and select PROPERTIES | HARDWARE | ADD NEW HARDWARE
Let the computer search on its own [recommended]
4. Right-click the desktop and select PROPERTIES | SETTINGS | DISPLAY
5. Select your external monitor
note: if you do not see multiple monitors listed, your video card may not support dual monitors
6. The numbers 1 and 2 indicate primary and secondary display. Display 2 is initially on the right side of Display 1
7. Right-click on Display 2 and select ATTACHED
8. Select the EXTEND WINDOWS option. Click Apply.
Check out this reference too: http://www.microsoft.com/windowsxp/using/setup/learnmore/multimonitor.mspx
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