FYI....
One common problem that occurs is that the operating system, such as Windows XP, never appears. In other words, the computer starts up but nothing appears on the monitor. Assuming that you have already ruled out the monitor as being the source of the problem, your next step should be to take a look at the video card.
This particular problem can be caused from several different things such as a video card that is not properly seated or a loose connection from the video card to the monitor. Troubleshooting this problem will require you to locate the video card inside your computer and verify the following:
Assuming that the video card adapter is separate from the motherboard, you should check that the card is properly seated. Sometimes one end of the card may creep out of the slot when it is initially screwed in resulting in no display appearing on the monitor.
Verify the correct jumper settings for video cards that are mounted to the motherboard. This will require you to check the documentation that shipped with the hardware.
The cable running from the monitor to the video port may also be the culprit. Examine the monitor cable to ensure that there are no broken or bent pins. A bent pin can usually be straightened using a pair of sharp-nosed pliers. In the case of a bent pin, you will need to contact the manufacturer of the monitor to determine if the cable can be replaced. Also check that the cable running from the monitor to the VGA port is secure. Although these may seem like simple troubleshooting steps, it is often the simple ones that people overlooked.