First of all, did you install any other software for codecs? Codecs are coding and decoding software for audio and video. They work with your video and sound cards, along with XP to ensure when you go to listen to certain types of audio, and watch certain types of video, you're going to see what you click on.
Also, check to make sure you've got all your volume controls for "Master Volume", "Wave/MP3", "Line Out", and "Front L/R" up to your comfort level. Check to see if digital audio is enabled. If you're not running anything from fiber optic cables, it shouldn't be applicable. Make sure all your speaker connector points and wiring connections are plugged in where they should be. You can still get audio on rear channels for other types of things, especially if DSP or surround mixing are on.
In my opinion, try to see if you'd changed any of the audio settings to 32-bit or IEEE Floating point integer. If that doesn't work, I'll try and answer this after I come back home, getting off of work.