How is it connected? Using all of the audio ports? What color? Just the green? Depending on how its connected, its could be nothing more than how its configured in windows. IE, a 2.1 setup with only 1 connection to the PC has to be configured as "stereo" with no sub. There's a lot of variables here.
You need to verify your audio settings. Go into your control panel and go to sounds. Right click on the speakers in question and choose configure. Make sure its set to stereo. Even if the speakers have a sub, it won't see it that way. You may have to check other sound settings as well. My mobo has RealTek, so there's a RealTek manager option in control panel is well. Make sure it's set the same way.
I just went through something similar. One PC has 5.1 system, the other 2.1. Tested my 2.1 speakers on the 5.1 PC. No bass from sub because it was set up for the 5.1. Once I changed it to stereo, it worked. Could be something else, but its a good place to start.