Almost certainly you need to re-install the soundcard driver which has been wiped off when you re-installed XP. While XP is able to detect and install it's own drivers for many devices, it does not have drivers for everything -- especially devices which are newer than XP itself which is now some 5-6 years old.
If your PC was supplied with a 'Recovery Disc', that would have had all the necessary drivers on it which would have been installed by default whenever the recovery process was run. This does not happen when you use a regular XP setup CD.
Before we can help you we need to know either make/model of PC or make/model of motherboard (not the chipset, that's no help with sound drivers since several motherboard manufacturers use the same chipset but a different sound device; or the current owner may have fitted a better one.
If it's a big name PC like Packard Bell, Compaq or Dell, they carry drivers on their respective websites (just select your PC model). If not a big name then download 'Belarc Advisor' from here:
http://www.belarc.com/free_download.html
Run it to produce an audit of your PC specs. and post it to the forum.