Go to start, run, type in dxdiag (Direct X Diagnostics) and let that gather the info. Once it has, click the sound 1 or sound 2 button (I have a sound card and a mobo supplied thing so i have one under each) but just look under those, and see what it says. it should say something like...
Realtek HD Audio Output or SB Audigy Sound Output
Something along those lines.
Once you figure that out, grab the drivers for it and make sure its for the right operating system (XP 32 bit , 64 bit, etc)