Well - a number of installation routines I've seen--most recently something
that I ran today on a Mac G5--can't recall just what--recommend that you
disable antivirus real-time scanning during product installs, and I can say
that I have seen reports of issues with that.
The recommendation is to do this just during the installation of the
product, and not afterwards.
I don't normally remember to do anything special during installs, and have
installed Microsoft Antispyware on dozens of machines without taking that
precaution. There was one post here, however, of a perplexing install issue
which turned out to hinge on a misunderstood blocking prompt from a
third-party product ocurring during the Microsoft Antispyware install.