It doesn't seem you did something wrong. Although, maybe you did. You bought a sound card without Windows 98 drivers, or you pointed to the XP folder instead of the Win98 folder when asked for the folder on the CD the drivers were in. Or there is an internal sound device on the motherboard which you should have disabled in the BIOS before installing a 'real' sound card. To name 3 possibilities.

How to undo? That's the first priority. I see three different ways.

1. When in safe mode, go into Device Manager, and delete the sound card. That will remove these erroneous drivers. Then remove the sound card from the machine or Windows will detect new hardware and prompt for the CD again.
2. If you want to let sound card in, disable it in Device Manager (in the 'current hardware profile' or something like that). Then the card and the drivers are ignored by Windows, and you should be able to boot normally.
3. If both ways above don't work (although I see no reason why they shouldn't) remove the soundcard, boot via the boot menu in MS-DOS, type scanreg /restore (followed by enter). This gives a menu of 5 recent backups of the registry (made every morning). Choose the last known good one and let the program restore it. This will effectively undo the install of the drivers (although the files will still be present on the hard disk).

Kees