Make sure you’re reinstalling the correct drivers, there are actually various AC97 is just a standard, make sure you’re installing the proper drivers.
Are you using onboard sound or are you using a separate sound card, and who makes the motherboard/card?
Also, this may sound odd, but make sure that if it is onboard sound that’s it’s enabled in the BIOS.
That's a good question because if it is a separate card, it can be removed and then reboot the machine then shut down and reinstall the card. Windows should find it and either load drivers or ask for the CD with the proper drivers. Either way you should have sound.
Or...if you have onboard sound...buy a decent sound card that comes with its own driver CD and install that in the same way.
Or...just save your important files and wipe the thing clean and reinstall XP. That would be the fastest route to Nirvana.
Make sure you have your valid 25 digit COA license key ready before you start that routine.