Why can’t I change screen area size

I have proview 15inch pro-500 on s3virge-dx/gx pci(375/385). I could change screen area to 800X 600 before. But my screen is now 640X480, whenever I tried to change to 800X600, Windows 98 SAYS: THE DISPLAY CONTROL PANEL CAN’T CHANGE THE DISPLAY SETTINGS, PLS CHECK YOUR DISPLAY HARDWARE IN DEVICE MANAGER. I reinstalled the drivers for monitor and video, but still could not change the setting. Pls give me some suggestions. Thanks, Mei