Some people got confused between antivirus and antispyware products.
Usually, they will only do what they are designed to, ie Avast gets viruses, WSS gets spyware.
Therefore it is not right to substitute one for another, as they are two totally different things.
Chris is spot on.
I think that Avast! will get some spyware, but you ought to have a program that speicializes in Spyware. The all-in-one solutions aren't so hot.