Could you give some more information?
What operating system are you running?
What make and model video card is the new one? PCI card or AGP?
Does your motherboard/system support the new card that you bought? Some newer motherboards only support specific makes and models of AGP video cards (for example, AGI types).
Did you uninstall the drivers for the old video card before installing the new card?
Significant changes in the type of video card that you switch out, including different makes can cause this, especially if you don't rid the system of the old drivers.
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"I love it when a plan comes together." - Hannibal Smith