Microsoft has released beta 2 of Windows Defender, the successor to Microsoft AntiSpyware (beta 1). It installed well on my machine, although I removed MS AntiSpyware first. Happily, neither the removal nor installation required a reboot.
Now, I've got a problem. Windows Defender will not update itself. It fails with an error 0x8024402c. Buzzing around through Microsoft's site, it seems that this error is related to Windows Update. My current theory is that because my PC is set to update through a WSUS server, Defender is looking in the wrong place for its updates.
Update: Feb 22:
For any folks using WSUS, you must add "definition updates" to the types of things that are synchronized, then set "definition updates" to "automatic install" to have them pushed out to clients. Now that we understand this, we've put Windows Defender on several computers on the LAN and it seems to be working well.