Navigate to the key below using Regedit.exe and change the value of "NODRIVES" to one of the settings below.

Key:[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
Value Name: NODRIVES
Data Type: REG_DWORD
Data:A bit mask of 11111111111111111111111111 (0x3FFFFFFh) would hide all drive letters.
More info: http://support.microsoft.com/support/kb/articles/q158/4/57.asp