I think the best tests are run by users.
I have installed Spybot and Adaware on several machine nearly 20, for years and have compared results. Company machines used by employee go many places!
I also have been running MS program on my machines {will not download beta version anything , on someone elses machine and, some are 98X}.
My experience is, for what ever its worth, AdAware out performs the other two.
I have always made habit of running Spybot first and then AdAware--- Adaware finding what SB missed. Now I have reveresd that habit and SP finds nothing. Not bad speaking of SB , but just what happens weekly.
MS ? I have been running since begining, never found anything at all, and I run it first {only two machines}.
I have read that some are finding serious problems that have been missed, so so maybe it is effective , just haven't been hit with the right problems.
I do know that recently AD has removed REG problems {not simply tracking cookies } that have missed by the other two programs

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When we speak of being "clean' all the time, that may be the case ,depending on habits { I don't go to porn , free stuff , P2P , ect.} so have little problem---but MANY do!.
My point is , if I was limited to one scanner { great that its not the case!} it would be AdAware.