If you have updated to Windows 8 you might have had difficulties installing Fastboot and adb drivers for Xperia devices, here a way to do it thanks to aslike2 on XDA:

1. download flashtool and install
2. open cmd as administrator and type “bcdedit /set testsigning on”
3. restart PC
4. install driver in the flashtool “drivers” folder, check fastboot, agree when prompts
5. type “bcdedit /set testsigning off” in cmd and restart PC
6. go to “x10flasher_lib” folder flash whatever you want