You should be running Windows Defender in addition to an antivirus program.
The program continues to be free. If you’re currently running Microsoft Antispyware, you should be able to upgrade simply by installing the new version. (There’s no pressure to upgrade Microsoft Antispyware immediately. Feel free to wait a month while initial bugs are ironed out.)
The interface has been redesigned to match Windows OneCare Live, but the two programs are not integrated.
I experienced a problem that other people have reported in the support forum – after installation, Windows Defender claimed it was “unable to update.” The solution was to visit Microsoft Update and obtain “critical updates” that included updates for Windows Defender. After rebooting, everything was normal.
In this release, Windows Defender does not display an icon in the system tray by the clock. There’s no indication that it’s running unless there’s a problem, in which case various things appear to alert you. I find it a bit unnerving not to have the reassuring icon, but it’s not a bug; many people believe the system tray should be used only for items that need attention. The designers have already listened, though, and the icon will become an option in an update soon.