You need to change the sound settings. Open control panel. Make sure the view is set to small icons. click Sound. Right click your sound card and select Configure speakers. You should select one of the boxes under Full Range Speakers. If it doesn't work after the speakers are properly configured, then double click your sound card and you be able to change more options from there. You might also have an icon for you sounds card in control panel that options can be changed through. The main thing to look for is a 2.1 option.
Based on what you have written under you system specs, it looks like you are using the basic default Microsoft drivers
for your sound card. Installing the latest drivers from you manufacturer would help solve any sound issues.