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There is also notice that Windows Defender (Beta 2) expires December 31. It would be a good idea to update your copy before then.
It is recommended that you uninstall the beta version before installing the new version. I first tried installing my new copy of Defender over the beta product, and I was forced to reboot to complete the installation. It probably would have been best to uninstall the old version first. Given that Defender ties into IE7 and Windows Live OneCare a reboot is probably necessary for most setups. Other anti-spyware products often require a reboot as well.
I ran Spybot on a machine with Windows Defender. Spybot found hundreds of issues that defender apparently missed.
It's good to have AdAware and SpyBot in your arsenal as well, but don't discount Defender.