|Right click an empty spot on your desktop. Choose screen resolution> Advanced settings> Properties> driver> uninstall. Follow the prompts. Then install the correct driver for the adapter you have.|
These instructions are for Windows 7. Hopefully they are the same for Vista.
Before doing that be sure to have the latest driver for your display already accessible.
For best results you should always run your screen resolution at the native resolution, which is 1280x800.
Get the correct drivers for your computer at the link below.
If you unplugged the monitor while under power you MAY have damaged the GPU. Does the display look OK when reconnecting the external monitor?