I just reinstalled MiKTeX basic 32bit on Windows 7 64bit. Beforehand, I had thoroughly cleaned up the remainders of the previous installation. The installation went smoothly, but right afterwards, I wanted to update the packages, but I got an error "The operation could not be completed for the following reason: Windows API error 5: Zugriff verweigert. Details: C:\". After changing the permissions as described in http://tex.stackexchange.com/a/78674/4012, everything worked fine.