Equalizer APO might be what you're looking for.
After downloading and installing (and restarting Windows), however, it will take a bit of configuration to achieve what you need:
- On first start, it will ask you to configure the output devices. Select your main output device, usually
Speakers - VIA HD Audio or
Speakers - Realtek HD Audio or something like that.
- In the Equalizer APO Configuration Editor, click the red minus icons next to each of the active filter items to remove them all.
- Click the green plus icon, select
Copy (Copy between channels)
- Drag the red
L over to the green
R below it, which will create an arrow pointing diagonally from one to the other. Repeat the same for the other side, dragging the red
R over the green
If the power icon for that configuration is white, it means your stereo has been swapped. If it's black, it's off. Run some tests to verify this.
This is a simplified, GUI version of the manual configuration task, originally suggested at Seven Forums.