This can occur when another application (sometimes security applications or "cleaning" 

applications, such as CCleaner) changes a registry value in your system.  



The value

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders\Local AppData



can be changed from



C:\Documents and Settings\-YOUR USERNAME-\Local Settings\Application Data

to

C:\Documents and Settings\NetworkService\Local Settings\Application Data



by another application and therefore MM appears empty.



To fix it, go to Start -> Run, and type regedit

Then, find HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders\Local AppData

Double click "Local AppData" and change the value to C:\Documents and Settings\-YOUR USERNAME-\Local Settings\Application Data