On the Mac, under the "Sharing" pane of the System Preferences, enable "File Sharing" and then visit the "Options..." and enable the SMB sharing options and select a Mac user account.
Then, go to the PC, and type in the URL bar of an explorer window:
Code:
\\ip-address-of-mac
...where "ip-address-of-mac" is the IP address of your Macintosh. When prompted for a username and password, enter the username and password of the account you selected in the "File Sharing" procedure on the Mac mentioned above.