I've noticed that this update for Vista actually re-creates/re-modifies all
the files under C:\Program Files\Windows Defender, which are necessary to
run Defender. (The update also resets files under
C:\ProgramData\Microsoft\Windows Defender\Definition Updates\Default). In
other words, it appears to essentially re-install Defender for Vista:
http://www.microsoft.com/downloads/...5F-A299-475A-95A0-5A5447AE284B&displaylang=en
Too, the update also creates two new folders in C:\Windows\winsxs, which
would be used by the System File Checker utility.
I would try downloading and installing the update, first. If Defender still
doesn't work, try the System File Checker, by right-clicking on Command
Prompt (Start Menu\Programs\Accessories), and choosing Run as Administrator.
At the prompt, type the following:
sfc /scannow
Press enter on your keyboard. The scan may take awhile.
