Default [Q] Problems Unlocking bootloader HTC Sensation XE

Hello, I've tried several times to unlock the bootloader of my HTC sensation XE from HTCdev website. I am stuck at step 4 when I try to run fastboot.exe. It says my PC is missing the file ''AdbWinApi.dll''. The error literally says: ''The program cannot be started because AdbWinApi.dll is missing on your PC. You might solve this problem by re´nstalling the program.''
I did 'install' Android SDK, however it's not really installing because the program just starts up and does not install. I have watched other threads about rooting the HTC Sensation XE but back in the time the Android SDK was different from what it is now so that's why I don't know what to do to fix this problem. Does anyone know what I should do ?