Many of you may not know that by default Windows has something called "Administrative Shares". They are enabled as soon as you install windows (except for Windows Vista and 7 home) and if you are able to authenticate as a member of local Administrators group you will have access to every drive on the system.
This can present a security issue for some as they are not very useful and open up a fairly big hole if someone ever was able to authenticate. If you are inclined to remove these the method is fairly simple and you can write a batch script if you wish using the following commands:
NET SHARE C$ /delete
NET SHARE D$ /delete
NET SHARE admin$ /delete
Other methods of prevention are removing the administrators from the security tab of the drive, disable file and printer sharing (think public network). Or encrypt the filesystem :P