It sounds like it didn't figure the right graphics card or you changed something on the graphics portion of the install. You should be able to install with the cd's again, using the UPGRADE option instead of the full install, and correct the settings.
Other than that , you can of course try to manually fix the config files...........but that isn't for the faint of heart....

Not impossible, but upgrading and having the installer on the cd do the work is much easier
HTH's