You need to disable driver signature enforcement. On Windows 8/8.1, Press WinKey+C, goto Settings => Power, hold Shift and click Restart. Then, choose Troubleshooting => Advanced options => Startup Settings. Then your computer will reboot and you'll have a menu where you can select "disable driver signature enforcement". After that, install the drivers normally, they will work this time...
Sony Xperia M dual [C2004]
ROM: 15.5.A.0.18 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)