vista: Share a folder or file directly from your PC:

Windows Vista improves on the Windows network folder sharing experience first introduced with Windows XP by giving you more flexibility in what you can share with other people and improving the setup process. With Windows Vista you can now share folders and individual files with any other user on the same corporate network. From any explorer, select a file or folder and on the command bar choose the option to Share. Enter the name of another user on the same network, and give them appropriate rights of access—reader, co-owner, etc. To help close the loop, Windows Vista can even automatically compose an e-mail to the individuals with which you have shared the content. The auto-generated e-mail contains a hyperlink to the shared content, enabling the recipient to instantly be taken to the shared content.
Source: Microsoft