Hi. I hope i can help you.
When Windows boots up, it takes control of the video, so what you get before Windows starts has nothing really to do with what you get after Windows boots.
Open your video settings in Control Panel. Make sure it's not set to minimum brightness. Also check the power settings. It may be powering down when it is not supposed to.
Check your battery. If you are running on a low battery, the screen will automatically go dim to save battery power.
If that's the case, run the machine off AC power.
If all else fails, download and install the latest video drivers for your machine. You may not find Windows 98 drivers on the mfr web site, but you can still get the drivers if you go to Tom's Hardware Guide, or try Google the name of the video card and drivers for Windows 98.
Hope this helps.
FW