By default OS X now uses SMB for connecting to networks. This makes sense as Mac, Unix/Linux, and Windows can all talk SMB with OS X.

Windows XP does have default shares, as was rightly said earlier on, it can also have its shares set to not need any authentication to access them. This is something that so many people forget about. It could be that these windows users have not set there permissions up correctly for their shares and this has left an entry for 'everyone' which is set to 'full access'.