How do I stop the default admin shares from being created?

A. This can be done through the registry.

This can also be done using the policy editor. Start the policy editor (poledit.exe), load the default computer profile, and expand the Windows NT Network tree, then Sharing and set "Create hidden drive shares" to blank for server/workstation.

There are a few other options though. The first is to use NTFS and set protections on the files so people may be able to connect to the share, but they will not be able to see anything. The second is to delete the shares each time you logon, this can be done through explorer, but it would be better to have a command file run each time with the lines
net share c$ /delete
and for all the other shares, however these shares are there for a reason so your machine can be administered by the servers, so if you delete them system managers may have something to say about it!