Apple has done a lot of work to make SMB (the Windows file sharing protocol) work in OSX. I've never actually done this myself, but it should work very well. Making sure Macs can live in a Windows world is one of Apple's biggest priorities.
http://docs.info.apple.com/article.html?artnum=106471
You can set up a share on your Windows machine, then access it from the Mac. This will allow both machines to read/write to the share.
Alternatively, you could format an external hard drive with HFS+ (Mac filesystem) and install this on your PC:
http://www.mediafour.com/products/macdrive/
I'd go with the first route and only do the second one as a last resort.
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