You can hide disk drives from the users, using a registry key entry. The value that controls drive hiding is actually a bit mask. HKCU\software\microsoft\windows\currentversionj\policies\explorer\NODRIVES is a REG_DWORD which makes it 32 bits long. Since the system can map 26 drives, this mapping works out very well. The upper six bits are ignored, and the remaining 26 bits map to a drive letter, with A: in the right most position and Z: in the left most. For example
00000000000000000000000000
ZYX CBA
To turn of drives A, B, C, D you end up with a mask value of 00000000000000000000001111. To turn of all drives, use a 1 in all the bits.
There is only one problem, if your drive letters change, say when you install a new hard drive, your NODRIVE values are shifted and you may find that a drive has vanished.