Somebody's asking about Windows Defender, and now about Microsoft Security Essentials.
Do NOT mess with Windows Defender! It is partNparcel of, it is within, Windows OS. If you jack with it, you just may cause Windows OS to be invalidated, unauthenticated. You can install, uninstall, meddle with MSE all you want, however stay out of the registry. 'Nuff said?
[I suspect if somebody does not heed the above, their next post will be about their Windows OS...]