The 2009 McAfee, Norton, Symantec, etc. anti-malware suite product for
Windows Vista disable Microsoft Windows Defender to improve system
performance. Windows Vista users who have installed these products should not
enable Windows Defender
If that 'error' message appears, disable Windows Defender from startup
(Start \ Run \ msconfig),
or if you wanna enable Windows Defender, go (Start \ Run \ services.msc) and
then turn off automatic scanning (in Windows Defender)