stereo recording using Behringer Q502USB does not work

Hi, I recently bought a Behringer Q502USB mixer and want to record a stereo audio input signal on my windows vista computer. I do get a signal on both stereo channels, but the L/R signal is identical. Also when I plug in a microphone, the panning only works to the left, but panning to the right reduces the signal to zero. Both results indicate to me that the USB signal is NOT stereo, or that my USB sound input is set in a wrong way. How can I solve this??
Thanks for helping out!

The obvious one is to make sure Audacity is set to stereo next to the microphone symbol in the main work window.

The not-so obvious one is to make sure Windows is set for stereo. You do that in the Windows control panels.

Start (lower left) > Control Panels > Sound > Input (from fuzzy memory)?