Why is Spybot-S&D that fast?FAQ #001
There are different ways to search for spies. One would be to search the complete registry and harddisk for suspicious entries and files. That can take a lot of time.

Spybot-S&D takes advantage of the fact that all spies have to anchor themselves at a few places over the system to get active. It starts to search at this places, following the information gathered there to catch the whole spy.