Every time you connect an Android device to the computer, the system installs the available drivers. It’s most likely that when you previously installed the drivers and a specific Android device you missed the warning and accepted it. Windows gives this error in order to prevent the bad installation of the drivers and to prevent malware because it is a wide-spreading technique and also a very effective one. In your case you should accept the warning and install the driver anyway. If you want to get rid of that message for future reference you might want to try and right click the My Computer > Properties > Hardware tab > Driver Signing and choose "Ignore". It’s not recommended but will help you get rid of the error message in the future.
