do you mean you are installing XP and after you do the sound doesn't work.
If so you need to download the sound card drivers for your sound card.
If it's a branded machine then goto the support section on the manufacturers website and find the drivers for your model of computer and download all the relevant drivers, if you have problems post the make and model of your 'puter and somebody will point you to the right place.
You will probably need
Sound card
network card
video card
modem
chipset
errr, that will probably cover it :)