OK. Running XP SP2 with all updates applied. Defender service stopped a
couple of days ago (maybe after the last Windows updates). I could restart it
but then it would stop again a few moments later.
I gave up on Defender and tried CounterSpy and its service stopped as well!
Very odd. Then I received the regular mail from Fred Langa pointing to
ongoing svchost.exe problems and followed his suggestions:
Step 1. Patch msi.dll. Users need to replace the msi.dll file (the Microsoft
Installer DLL) that svchost.exe controls. To do this, download and install
the patch from Knowledge Base article 927891.
Step 2. Update Windows Update. Users also need to get the latest Windows
Update client, which is version 3.0. Links to the 32- and 64-bit versions can
be found at the Windows Server Update Services (WSUS) Product Team blog.
For me, Step 2 turned out to be unnecessary as I already have the latest
Windows Update client. Anyway, after rebooting, the CounterSpy service is
working so the problem appears to be fixed. I'll probably stick with
CounterSpy for the time being.