windows defender v microsoft security essentials
Windows Defender on XP/Vista/7 detects and removes known spyware only. It is not designed to protect against the full breadth of malicious software, and specifically does not prevent viruses, worms, Trojans, and other malicious software from infecting your machine.
Microsoft Security Essentials should disable Windows Defender on Vista and Windows 7 and uninstall it from XP. In some cases, this does not happen automatically.
Microsoft Security Essentials is designed to disable Windows Defender in order to manage the PC’s real-time protection, including anti-virus, rootkits, Trojans and spyware.
Windows Defender on Windows 8 is more like MSE than like Windows Defender on older versions of Windows.