I downloaded, installed and am currently running Microsoft AntiSpyware Beta (the one they acquired via Giant Software). The installation and function seems to be straightforward.
Running it on my system however detected a "Spyware" threat -- RealVNC(which I legitmately run for controlling remote servers). I suppose it is a potential threat(if it were running a server) as it could allow remote control of your computer. Strangely enough though similar programs from Microsoft (Remote Desktop, NetMeeting) were not detected or mentioned even though they would offer the same threat level. I don't know if any of the commercial programs allowing remote control would show up as "threats" either.
I guess there is a thin line between potential threats and legitimate programs in some cases. I wonder where and how Microsoft draws that line? Based on commercial interest perhaps?