Short answer. Local Group Policy.
Go to Start -> Run and type gpedit.msc in the Open box and click
OK.
In the User Configuration branch navigate to this location:
Administrative Templates\Windows Components\Windows Explorer
In the right hand pane take a look at the following policies:
Hide these specified drives in My Computer
Prevent access to drives from My Computer
Right click on each entry, select the Explanation tab and see
which one best accomplishes your goal.
Keep in mind that all users including Administrators will be
effected by this policy unless you follow the procedure outlined
in this article:
http://support.microsoft.com/default.aspx?scid=kb;en-us;293655
An alternative approach, if you've disabled Simple File Sharing,
is to deny the Read permission for the Administrators group to
the %systemroot%\system32\GroupPolicy folder. The only drawback
with this approach is that you'll have to remove the deny every
time you want to change any group policy settings. Here's an
article that explains the procedure in more detail:
http://www.theeldergeek.com/gp07.htm