1. Start Registry Editor (Regedt32.exe).
2. Locate and then click the Default value under the following keys in the registry if this problem occurs with folders:
HKEY_CLASSES_ROOT\Directory\shell
If this problem occurs with drives:
HKEY_CLASSES_ROOT\Drive\shell
3. Click Modify on the Edit menu.
4. Type none in the Value data box, and then click OK.
5. Quit Registry Editor.

http://support.microsoft.com/?kbid=321186