Sounds like a simple case of not having the right drivers.
Try going here, to
OmegaDrivers. They work better than the actual ones that ATI made, not to mention they can be uninstalled SIGNIFICANTLY easier than ATI's.
ATI makes horrible drivers...
If you are doing this in linux instead of a windows environment, let me know.
Also, once you HAVE your new drivers, disable your on board video card under the manage computer screen.