Ages III, black screen

When I try to change the graphics from 800 x 600 to anything else the screen
goes black. I can still hear the sounds. I haven't played it for a while and
I know I had it at a higher resolution before but don't remember how how I
did it. When the screen goes black the only way I can turn the game off is to
restart the computer. Retarting the game still gives me a black screen, I
have to uninstall and reinstall to get the game back.