Sharing a folder between mac and PC

So my girlfriend and I both have a laptop, hers is a mac with OS X, mine is a Fujitsu with Win XP.

We would like to share a folder between our computers. It would be preferable if we could use the internet for sharing (so we could both still access the folder while away) but we are on a home network so I suppose we could use that if there were none that worked over the internet. It is important that it is an actual folder accessible on our computer as a file system folder, what I mean is that I don't want to have to navigate to a web site to access or edit files.

Does anybody know of any programs or services that can do that?