"Windows Defender" and "Win 7 Defender" are two different things. The "Windows Defender" in Control Panel in Windows, is Microsoft's default anti-virus program, which is installed along with Windows, and when you update service packs, and so forth, it usually gets re-installed and reactivated if you have manually turned it off at any stage.
"Win 7 Defender" though, is a fake program by people pretending to be selling an 'upgraded' version of Microsoft's antivirus program, so if it is called "Win 7 Defender", then it is a fake virus program.