Make sure the lead from the PSU is connected properly...also check the rating of your PSU...at least 300 watts is recommended
Try a reinstall of the drivers...remove the ones you've got from add/remove programs...when you reboot the found new hardware wizard will start...CANCEL out of that...don't install the drivers with the wizard...just double click on the d/loaded drivers and install them that way.
Click start...run..type dxdiag and click ok...go to display tab and make sure all acceleration is enabled..might as well the test while your there as well.
If it still doesn't work,depending on the age of your PC,you might need to update the BIOS and/or the m/board drivers.Updating the BIOS should be a last resort tho...cos if that goes wrong it's big trouble
Of course it could be a faulty board.