monitor problem

it 'could be' you have been set to 16 colors by a game program or something. Control Panel/Display/Settings tab/Color Quality.
but also sounds like it is using the fallback vga graphics driver, which means you need to download and install the latest version of the graphics driver for your system.
others here can help if you post whatever info you can find stamped on the card, or if it's onboard video, what motherboard numbers.
if you watch the black screen with a warmed up monitor from a power off restart, the first thing on the screen usually is the video bios announcing itself. a clue. ati, a mfg, for example, has a tool find out what your card is. it is on their website. other mfgs have similar. so if you can get at least that much info, see?
post os version also?
if xp, have you considered trying a system restore point