http://www.novell.com/coolsolutions/trench/3600.html
Shannon Powers
As seen on: http://www.g4tv.com/screensavers/features/25208/Sarahs_Windows_Tweak_Hide_Your_Drives.html

1. Open Regedit.
2. Navigate to this string:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
3. In the Explorer key folder, create a new DWORD value by right-clicking Explorer, then choosing New DWORD value. Name the value "NoDrives" (without the quotes). This value defines local and network drive visibility for each logical drive on the computer. All drives will be visible as long as this value's data is set to 0.
4. Following the table below, enter the decimal number corresponding to the drive(s) you want to hide as NoDrives value data. When you right-click on NoDrives and choose Modify, make sure you select Decimal base, not Hexadecimal.

   Drive Number to hide 
   A: 1 
   B: 2 
   C: 4 
   D: 8 
   E: 16 
   F: 32 
   G: 64 
   H: 128 
   I: 256 
   J: 512 
   K: 1024 
   L: 2048 
   M: 4096 
   N: 8192 
   O: 16384 
   P: 32768 
   Q: 65536 
   R: 131072 
   S: 262144 
   T: 524288 
   U: 1048576 
   V: 2097152 
   W: 4194304 
   X: 8388608 
   Y: 16777216 
   Z: 33554432 
   All drives 67108863

5. If you want to hide more than one drive, you simply add the drive amounts together for a combined total. 
   For example, to hide the D:/ and T:/ drives, add the decimal value for the D:/ drive to the decimal value to the T:/ drive. 
   8 (D) + 524288 (T) = 524296
6. To disable all of your visible drives, set the value to 67108863.

You must reboot your PC to see your changes. Have fun hiding, you little sneaks!