The stock advice in these situations is to
1) make certain you are installing as an administrator on your machine.
2) try the control panel, add or remove programs, Microsoft Antispyware,
change, update procedure.
3) if this fails, try uninstalling and reinstalling.
4) Some have reported success by choosing a non-standard install
directory--say, c:\msas as an example.
(others report that none of this works!)
I know each of you in this thread has tried some combo of these, but these
are the home remedies that I'm aware of.