Defender won't run at startup

I've just noticed that Defender isn't running, even though the Registry key
(HKLM\Software\Microsoft\Windows\CurrentVersion\Ru n) is there. The path
specified by this key (%ProgramFiles%\Windows Defender\MSASCui.exe -hide) is
valid. I can copy and paste it into a Run box and the process appears in the
Task Manager (and stays there until I reboot). If I view the System Startup
page in Spybot S&D the key exists. I've actually changed '%ProgramFiles%'
to 'C:\Program Files' (without the quotes) to see if it makes a difference -
it doesn't. If I view the Startups tab in The Ultimate Troubleshooter it
finds the key but shows File Not Found in the Launch Command field.

I'm at a bit of a loss. I thought perhaps the permissions had got screwed
up for either the Windows Defender folder or the executable itself, but if
that were the case, should I be able to run the command manually? In any
case, the permissions can't be edited. Everything's greyed out.

I'm quite happy to run without Defender; I'd just rather be the one who
decides. I'm concerned that to all intents and purposes it's supposed to be
running but isn't.

Any ideas?