I have tried Spyware Blaster. It is mainly good for blocking or detecting it's known spyware.

More specifically I need to be able to block/or at least be alerted to, any modification of
HKLM\Software\Microsoft\Internet Explorer\Extensions or the normal BHO installation
registry key, so that third-party software cannot be installed in either location.

I have disabled the usual 'Enable third-party browser extensions' option in IE. but I think that just works at the 'Browser Helper Objects' level, because third party extensions can still install at the above mentioned registry key.

Thankyou for all help so far provided.