Respected Sir, I installed an nvidia 8400gs graphic card a year ago..........it works fine ....then two weeks ago i dusted up my cpu...........i inserted the graphic card then the computer wont boot up........the fans are working ..i can hear the sounds from the cpu ..the monitor would show amber light instead of green.....the computer would boot up normally without the graphic card........please help me!

It is best to disable the onboard video electrically via a jumper or dip switch on your computers motherboard or in its system BIOS. Please refer to your computers user guide or contact your computer manufacturer for instruction on this process.
In the event that your system does not support the above, you may be able to disable the onboard video card in the operating system using the instructions below.
Make sure that the new PCI video card is not installed in your computer.
Windows XP/2000
1. Right-click My Computer and select Properties.
2. Click the Hardware tab, and then click the Device Manager button.
3. Click View, and then Show Hidden Devices.
4. Click the Plus (+) sign next to the Display Adapters category to expand it.
5. Select the onboard graphics controller under Display Adapters and click Disable. Click OK to confirm.
6. Do not restart the computer until the controller is disabled.
7. Turn the computer off.
8. Install your PCI GeForce card and follow the included instructions from the card manufacturer.