First the obvious - double check your wiring to make sure both speakers are plugged in, and cables are undamaged. If they have a power switch, verify they are both turned on. One pair of speakers I own have a volume control on each speaker; verify the settings for each are the same.
In Windows XP; right click on the speaker icon near the time in the tool tray on the lower right corner of the screen, and select Adjust Audio Properties from the pop-up menu. In the dialog box that appears, click on the Speaker Volume... button just above the OK button.
You will then get a dialog box you can use to verify and change the volume for each side (there is no balance.) If both are at the same volume, try pulling up Windows Media player or some other application, and verifying that other applications are having the same problem. If all else fails; trying finding, downloading, and installing the latest sound drivers for your sound card.
(If you have Vista, someone else will have to help.)
-Best of success;
James