just as William said, it would always be safer to remove devicespecific drivers before changing things.... Who realy ever does? Anyhow, you can do this after the event:
When starting the PC, keep pressing F8 repeatedly when Windows is starting. This will let you enter the Start-menu. Select Safe mode and Windows should start in VGA-mode. In VGA-mode enter Display settings and reset these to VGA and standard monitor.
Restart the PC and now WIN98/ME should re-discover your graphics-adaptor and monitor and install the relevant drivers.