It sounds like you have a VGA monitor and when you plug it in to your motherboard VGA port it still works but when you plug it in to the VGA port on your video card it goes kablooey.
First, do a straight reinstall of your video card. If you have your original CD, great go ahead and turn your computer off. Take the video card out, turn the computer back on and make sure it's removed in hardware (Right click on My Computer, go to Properties, Hardware Tab, Device Manager Button and expand Display Adapters. Make sure your video card isn't there, if it is, right click on it and go to Uninstall.
Once that's all done. Shut off your computer again, and reinstall your video card all over again. If you don't have your original CD, luckily your monitor still works plugged into your computer. So run over to your video card manufacturer website (BFG, EVGA, etc.) and grab drivers. You can also grab more generic drivers from nvidia here
http://www.nvidia.com/object/winxp_186.18_whql.html
Once you have everything installed, restart, plug the monitor into the video card and see if it works.
If not, there are a couple things to try here. If you have access to another computer, plug the video card in there and see if it works after an install. You can also try a different AGP / PCI-E slot if you have one but that's probably not the problem. Also a long shot, but in Control Panel -> Power Options, make sure you have your monitor set to never turn off.