Windows Update error code 646

For Windows 7 and Windows Vista users

1. Click Start, and then in the Start Search box, type regedit.
2. In the Programs list, right-click Regedit.exe, and then click Run as administrator. If you are prompted for an administrator password or for confirmation, type the password, or click Continue.
3. Locate and then click the following registry subkey:
   
   HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
4. In the details pane of Registry Editor, locate AppData. The value of this key should be %USERPROFILE%\AppData\Roaming.
5. If the value is not set to %USERPROFILE%\AppData\Roaming, right-click AppData in the details pane, and then click Modify.
6. Enter %USERPROFILE%\AppData\Roaming into the Value data box, and then click OK.
7. Close Registry Editor.
8. Try to install the update again. If the problem persists, repeat these steps but use the following registry subkeys instead of the subkey that is specified in step 3:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
   
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
   
   
   
   
   
   
   
   
   Source: http://social.technet.microsoft.com/Forums/en-US/winserversecurity/thread/976cc6b0-327f-4fdf-904f-7d706dd3e985/