Just to be clear--no installation of Microsoft Antispyware is supported.
Installation on production machines is discouraged.
I've seen reports of high memory usage when installed on Terminal Servers,
and of alerts relating to on-the-fly printer installation on Citrix.
If I were trying to do this as a means to cleaning up a problem, I would
want to do it in maintenance time, and be able to log on as each individual
user, clean, and uninstall.
I wouldn't want to just install as an app for all users and leave it
running--too much risk for me.