There seems to be a problem with updating Windows Defender when using Microsoft Update instead of Windows Update in Windows Vista. The problem becomes apparent when Windows Defender puts up a warning in the system tray that its signatures are not up to date. This of course shouldn’t happen at all with Automatic Updates turned on. Manual attempts to update Windows Defender result in an error, while Microsoft Update insists that the system is up to date and no new updates are available.

According to the list of Windows Update 5 errors the above error code is 0x80240029 – WU E INVALID PRODUCT LICENCE: A product with an invalid license was found on the system. This is just plain silly (in my case it was happening on an OEM Vista Home Premium edition that came with my new computer) though a look in the WindowsUpdate.log file sheds some light on it:

2007-03-10 07:30:30:258 1144 1308 Report REPORT EVENT: {3B3B2164-091C-4856-9FBC-9D99EDAE3198} 2007-03-10 07:30:25:259+0100 1 154 101 {00000000-0000-0000-0000-000000000000} 0 0 Windows Defender Failure Software Synchronization Client has an invalid Pid.

Looking at this record you can quickly stop wondering why the Pid is invalid. An empty value instead of an actual one is used. Fortunately the solution is quite simple (once you know what it is): disabling the Use Microsoft Update checkbox in the Turn automatic updating on or off section of the Control Panel.

Once you do that Windows Defender will successfully update itself. Windows Update will also find its updates now and maybe even some other updates that didn’t show up when Microsoft Update service was used.