Why does Windows OncCare turn off Windows Defender?
If Windows Defender is installed on a machine and then OneCare version 1.5 is installed afterward, by default the installation will disable Windows Defender. Windows Defender will then display a dialog indicating that it has been disabled, and asking if the user wants to turn it on again. When you are installing OneCare version 1.5 you should close this dialog without re-enabling Windows Defender. OneCare intentionally disables Windows Defender because it can cause system instability to have both running at the same time, and since OneCare is a superset of Defender, there is no reason you would need both of them running. For more informaion, see http://windowsonecare.spaces.live.
