Depending on how you have them wired, can you not simply wire them oppositely. (ie swap the left and right speaker wires)?
If you've installed speakers that don't have easily disconnectable wires (and you dont wish to cut them), I don't think Windows itself has an easy way to reverse the speaker signals but some sound card drivers do.
So it depends on your sound card.
Good Luck.
(me I'd cut the wires and splice them back together... in reverse)