The answer is quite simple. Connect the machines together (or on a network, behind a firewall) and enable filesharing on both. Using the PC, you will see that the Mac already has a folder shared by default called "Drop Box". This is a folder that you can merely drop stuff into, to get files onto the Mac. This folder is located in ~/Public/Drop Box.
Now over on the Mac, go to the desktop and push Command-K to bring up a connection dialog. Inside that dialog, type "smb://" and then the name of your PC. For example, mine is "smb://cerebellum". SMB, stands for Samba which is a Unix/Linux program for connecting with Windows. Once you do that, it will connect and ask you which share (folder) you want to connect to on the PC.
That's it! Good luck.