Sharing folders is probrably one of the most common activity when you are on network. But as time passes you forget what folders you shared and if your folder is open to all i.e. without user protection then you can not find whos is accessing what.

This tutorial is in reference with 30 Days Windows Vista Tutorials and Guide

In Windows Vista  , Shared Folders under System tools of computer management allows you to find who is accessing what via open file view. You can find the logged in windows user name of that person and see which folder is being accessed.

Moreover you can also find which folders on your computer is being shared and you can drop right from here all of them

Find this tool under Control Panel > Administrative tools > Computer management > System tools . When you take a look into the shared folder you would see there are lot of drives which you did not share at all . Yes you are right. 

By default windows shares all drives under c$ d$ which can be accessed with your adminitrative password. This allows you to access your files without creating shared folders. 

Admin Shared Folders

Just in case if you want to disable this I would suggest you to read PC Shares Article on how to Turn off admin  shares of your drive. All you need is change a hack in registry and you are done.

Related : How to make shared content available offline in Vista