How can i install sound card driver ?

so i have a windows xp computer, service pack 3, lenovo thinkcenter, i got directx 9 and i have a sound card inside my central unit, inside my computer, so a guy who fixes computer installed a software called Realtek, it's for the audio card you know, anyway, i was cleaning my computer from some junk i downloaded and i accidently deleted realtek and so there's no sound, i tried to install it again but i don't exactly how and i finally installed realtek ac'97 but it still nothing, no sound and i tried VIA AC 97 and nothing, i was going to try soundmax and i couldn't install it for unknown reason, so i need help please i want my sound back i can't play gta san andreas and other games, i can't watch my movies, i can't do anything without sound