Windows 98 COMES with a Gravis Gamepad driver.

The 2-axis 4-button joystick option should also work.

In Windows 98 SE, go to Start->Settings->Control Panel->Gaming Options.
Click on "Add", then select either option from the list. Click "OK" and hey presto.

In other versions of windows "Gaming Options" is called something else. You should be able to recognize it by either a joystick icon or a joystick+gamepad icon.