Unfortunatly, there are no good solutions to this Windows Update error that
have been posted yet. Microsoft says the error indicates an update is in
progress, but rebooting and trying it again doesn't seemed to have solved
it for anyone yet, though you might try that first. Here's what I'd try
next, if you're a home user getting updates downloaded directly from
Microsoft or using Windows Update.
Start > Run > Cmd
to get to a command prompt,
then Change Directory to the Windows Defender program folder,
probably using this command or a variation for your Vista environment:
CD C:\Program Files\Windows Defender\
once there,
reset your Signatures and Engine to their default levels with this command:
mpcmdrun -RemoveDefinitions -All
Then try updating Defender again from the GUI - down arrow to the right of
? Help
or if that doesn't get you current you might try this KB:
http://support.microsoft.com/kb/923159/
The current version levels are:
Engine Version: 1.1.2405.0
Definition Version: 1.17.2444.9
Let us know how it goes.