Quoted:
I have a voodoo 3 video card.
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Well, there you go! You need to always download the latest video driver for your card. You probably got a compatible driver with your card under 95, but Win98 loaded it's compatible driver... which wont work for your games.
Therefore:
Download the latest driver for your V3/Win98:
[url]http://www.3dfx.com/downloads.htm[/url]
Install that, then set your desktop to the screen resolution and color depth you prefer.
Then try your game again.
Some games require the desktop color depth not to be beyond 65000 colors. Try that.
Some game keep video settings... and now that your driver has changed, the saved settings are not applicable. Re-install the game.
OS upgrades are ugly. I NEVER do them. Always back up data, format, reinstall OS, update hardware drivers, reinstall apps, restore data.
It's a lot of work, but it makes for a fast, clean, system.