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OK, I have been doing nothing different than I have been since I have gotten Windows 7.

Now all of the sudden Windows Defender is turned OFF and I cant turn it back on.

It tells me that it is blocked by group policy.

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It is likely that you have become infected by malware of some sort - it is very common for such software to try disbale tools that could remove it. Try booting Windows in Safe Mode and running scans with other known-good tools like adaware (by "known good" I mean don't use anything that you don't know the quality of or that you have had recommended by a trusted source - most anti-malware tools advertised on the web are actually malware themselves), or defender if it will start in Safe Mode.

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If you aren't in a domain environment, I'd suspect any unexpected "blocked by group policy" error messages as being caused by malware. A "known good" anti-virus program that you can easily install (from a USB drive) on the fly (even in Safe mode) is Malwarebytes' Anti Malware. – LawrenceC Jul 17 '11 at 13:39

If you are running Microsoft Security Essentials, you do not need to run Windows Defender. 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.

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.

If Microsoft Security Essentials did not automatically disable Windows Defender on Vista or Windows 7, you should disable it to prevent conflicts. Windows Defender cannot be uninstalled from Vista or Windows 7.

If Microsoft Security Essentials and Windows Defender are both running, your system may experience performance degradation and other problems caused by the conflict of two services providing real time protection simultaneously.

Source: Microsoft Social Answers.

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i haven't installed MSE yet. should i? cause i cant really pay for anti-virus or anything like that right now (laid off) – Azvarr Mar 30 '10 at 8:28
    
MSE is totally free as long as your Windows is genuine. I've been using MSE for a long time and I am satisfied with it. – Mehper C. Palavuzlar Mar 30 '10 at 8:53

defender is outdated and has been replaced by microsoft secruity essentials look to see if that process is running.

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correct. look to see if msseces.exe is a running process. – Keith Mar 29 '10 at 20:22
    
True, installing Microsoft Security Essentials should be the solution to this answer. Together with MBAM you have a great free arsenal to keep those computer clean. :-) – Tom Wijsman Mar 29 '10 at 20:45

Have you installed antivirus or internet security software? The newer ones seem to suppress and override Defender, since they are more complete solutions.

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I do run Windows defender in safe mode with networking so I can update defender and run a full scan, but I only do it once a month.

That does make sense that Security essentials blocks defender but it seems defender will catch all the adware or malware your PC may have?

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This is really a comment and not an answer to the original question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. Please read Why do I need 50 reputation to comment? What can I do instead? – DavidPostill Apr 22 at 10:18

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