Here is what I figured and did to get around it.
First, press win + I to launch the charm bar, and click Change PC Settings at the bottom.
Then, go to General, and scroll to the bottom of the page, click Restart Now button under Advanced Startup section. Remember to save all your work first before clicking it.
[Note that on Windows 8.1, the Advanced Startup option has moved to Update and recovery section under Recovery]
The computer reboots to a startup menu, in which click Troubleshoot.
And click Advanced Options at next screen.
Choose Startup Settings next,
And click Restart button.
The computer reboots and prompts another menu to choose. One of the items is to Disable driver signature enforcement. Press number 7 or F7 to continue booting to Windows 8 with digital sign enforcement disabled.
And then I was able to get the Windows Security warning window offering me to install this driver anyway.
I would suggest changing the UAC settings first to see if the Install Anyway window prompts out. The solution showing off in this post should be the last resort to try out. And make sure the driver you are about to install is safe, though not digitally signed.
Extract the USB_VCOM_Driver.rar file and leave it
After that Open Device Manager and leave it open!
Switched off your phone remove battery connect now on the PC, you should now see a MT65xx preloader in the Device Manager.
Right click on that and install/update drivers show the path where you extract the folder and click Done.
Now you can restore or flash firmware using SP Flash Tool...............
Also check my work on Google boot logo thread