I'm not sure what you mean by "Spybot S&D has a function that protects registries from changes". It doesn't as far as I can see. I can make changes to my registry and Spybot stays silent.
What it does do, is to protect certain areas of the registry that hold things like Run or Run Once, or it can stop the IE Home Page from being hijacked. But only if it is running in the background, (its Tea Timer service).
So, is that what you are looking for, or do you want something else.
In my view SpywareBlaster from http://www.javacoolsoftware.com/spywareblaster.html would do a better job. Its downloaded definitions "immunises" the registry from changes without running in the background. it is not even a scanner. Just keep it updated. SpywareBlaster does not need to remain open for its protection to be active!
Another utility I use, which is very small and unintrusive, is Startup Monitor from http://www.mlin.net/StartupMonitor.shtml
It is only 60kb and checks for any programs/applications that try to set themselves to "run at startup". It's very useful for those apps like Real Player, IM's or other things that try to flood your startup list with entries, and is also useful against trojans or any malware that attempt to set themselves to Run, or RunOnce, or load on startup. It works on all Windows except it has not been tested on Vista.
I hope that helps.
Mark