This is a Windows Installer issue that we've seen in various guises
regularly through out this beta.
Do you get any clue at all about what this "Prerequisite Install" dialog box
is talking about?
If you keep hitting OK or whatever, does Microsoft Antispyware eventually
launch?
There are a couple of general approaches to fixing this one: First--if you
can spot what it is trying to install, and have the installation media--and
want the application on your system--you can reinstall or do a repair
install of the app involved. If you don't want the app installed, you may
even want to do a repair or reinstall, and then uninstall it--just to get
things cleaned up well.
In the absence of installation media, or if you can't figure out what app is
involved, you can download and install the Windows Installer Cleanup
utility:
http://support.microsoft.com/default.aspx?scid=kb;en-us;290301
Here's the rub, however--you'll need to be able to spot in the list this
utility provides, the application that the prompt is referring to. I don't
know whether that will be easy or not. If you can spot it with certainty,
use the cleanup utility to remove the installer traces of that application
from your system, and that should clear up this conflict.