Do you have Windows Live OneCare installed? Having that will turn off
Defender by default.
Another possibility is that you have a corrupted WMI database if it's
Vista's Windows Security Center reporting incorrectly that Windows
Defender is not turned on. If that's the case you should see MSASCui.exe
and MsMpEng.exe as running processes in the Windows Task Manager, as well
as Windows Defender being a started service in Computer Services. If
you're seeing those indications that mean Defender is active, repair
instructions can be found in this post: http://www.microsoft.com/athome/sec...8150&mid=77fda0aa-8775-40f1-ba99-befb3eb58150