I have been unable to install the last two definition updates (1.14.1530.8
and 1.14.1525.2) both give the same errors (0x80070643 as shown in Event
Viewer).
If rather than using Windows Update or Microsoft Update I instead ask
Windows Defender itself to get the update then I get a different error as
follows -
"Windows Defender was unable to complete the update: 0x80240022: The wrong
diskette is in the drive. Insert %2 (Volume Serial Number: %3) into drive %1.
I have tried uninstalling Windows Defender, redownloading and reinstalling
it (several times now) and this makes no difference. I have also tried
removing all registry entries relating to Defender (before reinstalling it),
and I have also removed the downloaded definition updates so as to force
another download.
I have also tried the steps detailed in
http://support.microsoft.com/?kbid=915105
Note: Each time Microsoft Update does show the file has been downloaded
successfully, it is the installation that fails each time.
When you reinstall Defender you start off back at definition update version
1.00. If I 'hide' the two definition updates (listed above) in Microsoft
Update then it finds the next oldest one which is 1.14.1521.63 and it _DOES_
successfully download and install this version.
Note: my machine had until the two versions listed above, been updating
Defender for several weeks successfully (ever since I first installed it).
Currently my Windows Defender versions are as follows
Windows Defender Version: 1.1.1347.0
Engine Version: 1.1.1481.0
Definition Version: 1.14.1521.63
My machine is on a home network so it is not behind any SUS or WSUS server.