Method 1:
Let's try running the Windows Update troubleshooter and check if it helps.
Troubleshoot problems with installing updates
Method 2: Clean boot
Place the computer in clean boot and check for the issue.
How to perform a clean boot to troubleshoot a problem in Windows Vista, Windows 7, or Windows 8
http://support.microsoft.com/kb/929135
Note: Once you are done with clean boot, please follow the section ‘How to reset the computer to start as usual after troubleshooting with clean boot’ under ‘more information’ in the article to put your computer back to normal startup.
If the issue still persists, I would suggest you to manually download the latest definition updates for Windows Defender. ****
How to manually download the latest definition updates for Windows Defender
http://support.microsoft.com/kb/923159
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