hi,
this is one of the most common problem while downgrading from vista to XP.. get the driver of your sound device...now u can't directly install the setup it wud display no audio device... so do the following...
1. Righty click my computer...click the hardware tab..
2. click device manager... now you will see a yellow mark on the uninstalled audio device. right click on it and update driver from a specific loaction( where you have downloaded your audio driver ).

this would work...but remember to install the microsoft unversal audio architecture before this.

hope to get rated as fixya!!!
regards
bzoy