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What anti virus and Firewall programs are you using?
Your post is somewhat confusing. Does Defender open after the warning or doesn't it?
All anti virus programs that I know of turn off Defender when they are installed. Example: When you install MSE it will turn off Defender as it should. I would suggest at least to start off would be to remove all anti virus programs and installing MSE.
Microsoft Security Essentials - Microsoft Windows
As I have mentioned before it will turn off Defender because it is not needed. It wasn't worth much in the first place. Check and make sure the built in Windows 7 Firewall is running. No other Firewall installed.
Remember that you should only have one Anti Virus and one Firewall installed on your computer at one time. Feel free to ask any other question and let us know how thing are working out for you.