How to get a refund - Now not required thanks to Pluto

[Moirai]

Hi,

As a big fan of the Penumbra series and Dark Decent, I've just purchased MFP via Frictional's main page (http://www.frictionalgames.com/site/) using Paypal.

All well and good. I download the Windows version...only to find out afterwards that Vista is the base O/S requirement, and that the game installer won't even run unless you are using Vista base minimum.

I'm running XP. -_-

So, how do I proceed with getting a refund, given that at no point during the purchase was I informed of the above and now have a game I can't even install, let alone run?

Many thanks!

P.S. if there's a better place for this kind of support (e.g. email, etc), please let me know.

EDIT: Edited for clarity on the actual problem.