i need to install windows defender for win7

It's highly doubtful that anyone "needs" Windows Defender for Windows 7.

Microsoft has needlessly confused many of its customers by renaming products and using the same or similar names for different things.

Windows Defender for Windows 7 was an anti-spyware utility. It was not an antivirus utility.

From Wikipedia:

In Windows Vista and Windows 7, Windows Defender was superseded by Microsoft Security Essentials, an antivirus product from Microsoft which provided protection against a wider range of malware. Upon installation, Microsoft Security Essentials disabled and replaced Windows Defender

If you want Microsoft Security Essentials, go here: https://www.microsoft.com/en-us/download/details.aspx?id=5201

This is what its uses were:

Windows Defender in Vista/windows 7 is an Anti-Spyware Program, not an Anti-Virus Program.

You should have an Anti-Virus installed.

If you are using Norton, AVG, Avira, Microsoft Security Essentials, McAfee, etc the default action is for Windows Defender to be disabled, as those AntiVirus Programs have their own Antispyware Applications.

It is nothing to be worried about.

Using the two Programs at the same time can cause System conflicts, etc.

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If you are using a different Anti-Virus, please read this:

"How to Start/Stop Windows Defender service"

http://computerstepbystep.com/windows_defender_service.html

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If you wish to have extra protection against Malware use the FREE version of Malwarebytes AntiMalware:

http://www.malwarebytes.org/products/malwarebytes\_free

And to now change Startup Programs in Windows 7 without using Defender, use either of these Methods:

"How to use MSCONFIG in Windows 7"

http://www.netsquirrel.com/msconfig/msconfig\_win7.html

Or the FREE Autoruns Program:

"Autoruns for Windows"

http://technet.microsoft.com/en-us/sysinternals/bb963902