Windows Vista includes a "Share" button on the explorer toolbar, but it doesn't work the way people used to XP/Win2k expect it to. Instead it pops up a wizard to make things "easier".
By default, this share button will not let you choose the name of the shared folder. Even more annoying, sharing a folder within your user directory will have a weird share name like computerUsersgeekContacts, which is much more difficult to remember.
If you want to share folders the way you used to on Windows XP, simply right-click the folder, and choose the Properties option.
Now choose the Advanced Sharing button.
Now you should be able to check the box "Share this folder", and choose a share name, which will be accessible from \computermyshare, for example.
You can also click the Permissions button if you want to allow write access to this share, or allow access to only a particular user.