If it's a pre-built (Gateway, HP, Dell, etc.) you should have a driver CD that came with it. Pop it in, let it autorun and follow the directions. If you no longer have the CD, go to the company's web site and download the drivers for your model. You'll probably need chipset dirvers, video drivers, sound drivers, modem drivers, and drivers for anything else that came with the system.
I'll assume you meant you reinstalled W98 overtop of itself. If not, you can explain what you meant.
Anyway, try this,
Restart the PC and keep pressing F8 during the bootup. You want to start tapping F8 after the drives are recognized on the screen but before the Windows splash screen appears. A startup menu with 6 options should appear. Use the SafeMode option to start Windows. Once Windows starts in SafeMode do this,
Start>Settings>CP>System>DeviceManager. Click on the "+" next to DisplayAdapter. Now you should see the name of your video card appear underneath. Write down the name. Now right click on the video card and select Remove. Restart Windows, Start>Shutdown>Restart and let it boot into Normal mode. It will reinstall the video card.
If that does not fix it then you need to get the video card driver from the makers website or you should have a CD with the drivers that came with the PC. Post the name of the video card that you wrote down earlier in the process if you need help locating the drivers.
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