Windows 10 system Windows Defender can not start the repair method

The Win10 preview has been released for more than a month. After many users tested it, many problems in the Win10 system were gradually fed back and solved. However, although many problems have been identified, there are still many problems that have not been found, so users still need to actively feedback them.
Sometimes a lot of friends have encountered a very troublesome problem in the process of using the Win10 preview version. The problem is as follows: When using other third-party security software and uninstalling, I want to change back to the system. After Windows Defender, I was surprised to find that it could not be opened. After opening, the system will pop up an error prompt window, as shown in the figure below. How to solve this problem? Let's take a look at the solution.

1. According to the error message, the first thing we suspect is that the application corresponding to the Windows Defender service has been tampered with by a third-party security application. You can query the application corresponding to the Windows Defender service by using the “Windows logo key+R” to open the Run window and entering “services.msc”; or by using the “tasklist /svc” command in the Command Prompt window. program.
After querying, we can see that the application path corresponding to the service is “C:\\Program Files\\Windows Defender\\MsMpEng.exe”, and the service cannot be started by clicking launch, or click to display Run, but still can not be used normally, the error remains.

▼ In the "Windows Defender Service" service, we use the “tasklist /svc” command at the command prompt to view the service and program dependencies as follows. The service PID process number is 2664, which corresponds to the "MsMpEng.exe" file.

2, however, after finding the corresponding application, we found that it has not been tampered with by comparing it with the certificate of the normal “MsMpEng.exe” file.