Do you have the onboard graphics disabled in your system BIOS or at least have it set to go with whatever slot your video card is on (PCI/AGP/PCI-E)?It's a compaq with 512 megs of ram and the moniters native resolution is 1024 768. The thing is the comp used to work fullscreen but when I got a new harddrive and installed the OS shit started fucking up. The card does require extra power but the onboard graphics gave me the same crap. Also a 7 series is an upgrade if you just had onboard before. Also I'm on service pack 1 if that's any help. I'll try the drivers later.
Have you upgraded to the latest BIOS?
Is the monitor CRT or LCD? Do you have a second monitor to try testing? If not, odds are you can connect your card to output to your TV via s-video or component cables. You might want to try that (it will be very low res probably 800 x 600 if its standard definition tv) and see if it still does the same thing. If it doesn't then I'd say something is fishy with your existing monitor. Otherwise its probably a software problem.
Probably unrelated but some general advice. You -really- ought to upgrade to service pack 2. If you are only on service pack 1 that means you probably havn't been keeping your computer up to date with security fixes. Install service pack 2 and then head over to windowsupdate.com and get all of the latest security fixes.
Finally, 512MB in my opinion is currently insufficent for modern computing with Windows XP. It was fine as of like two years ago but nowadays programs as such memory hogs. I suggest you upgrade to 1GB or 2GB if you can and you'll likely see a nice performance increase. RAM is very cheap now, even old DDR which I'm guessing your PC probably takes.
Thanks for the help anyways