File Sharing has to be enabled on Mac OS X under System Preferences > Sharing and then the volume/drive/folder of your choice, and under Windows, the folder/drive has to be enabled via the properties window.
Also there are a lot of guides available on the world wide wet. Just use a search engine of your choice.
EDIT: how to find the shared computers
PS: In Mac OS X, in Finder under Shared in the Sidebar. In Windows (XP) under Explorer > My Network Places > Your Network.
to share your Mac to a PC enable File Sharing in Sharing Preferences and in the Options enable SMB sharing. then your Mac should show up in your PC's workgroup in Network Places.