Right, well this could be the blind leading the blind, :blush2: but in the absence of intervention from a proper techie, I'll have a go:-
Do you know how to find the Device Manager in Windows 98? I think, from memory, you right click the My Computer icon on your desktop, and you can select it from there.
Once found, locate the sound card from the list, then right click it, and one of the options should be to update the driver. Select that, and follow the prompts to install from the CD, which should be inserted.
Hopefully, it will find the driver, and that will sort things as far as the error message goes.
Did you first establish that the Sound Scheme hadn't been changed? Look in your Control Panel, under Sound. It seems strange that you get the Windows startup sound, but nothing else. Try selecting the default Sound Scheme and see what happens. Do this before fiddling about any more with the drivers, although it sounds like you may have to reinstall them anyway now. Don't worry, this shouldn't be a major issue, and any problems should be fairly easy to solve.