Try the following:
1) Make sure your speakers are working and contain a powered amp (you might want to connect a CD or tape player to them just to check).
2) Next, open your windows Device Manager (inside the system control panel), and make sure your sound card drivers are installed. Also, check the "Other Devices tab, and make sure you see no "ghost" unknown sound items. If any are there, delete BOTH the sound card item (if there), and the unknown item, and reboot. Windows should redetect and install it correctly (have your windows CD and sound card driver disks handy when doing this).