Sharing a folder from Windows 7 to a Mac on two different networks

Hello,

I would like to create a shared folder between my Windows 7 PC to my girlfriend's Mac. I've learned this is a lot harder to do since we're both on different subnets. Currently, we're relying on the file transferring service from Team Viewer. So is there any way to do this? Would I have to make a dedicated file sharing server? I have a Raspberry Pi that I've been dying to actually put to good use.

I apologize if this isn't the correct place to post this, since it is involving a Mac.

It seems like you could use Windows SkyDrive for this without any problems.

With SkyDrive you can send a link to a SkyDrive folder and give the client write access to that folder.

Though I am not sure of your exact requirements that is about the easiest way I can think of. :)