I had a virus and had to do a sys rstr and now my sound card dsnt work?

i believe i am missing the dll. for drivers, but i have downloaded and installed all the drivers and software but wont work, btw i have winXP and realtek ac97, i even tried a new sound card, bought creative sound blaster and installed but had error and it couldn't complete install, and when i try to adjust volume it says no mixer installed, what do i do?