I was having issues with unlocking my bootloader. I installed the Android SDK properly, but no matter what I did, I could never get fastboot to work. I always got an error message saying "adbwinapi.dll is missing". I searched online and came across these two threads:
which unfortunately didn't have the right solutions
What finally worked for me was copying adb.exe, adbWinApi.dll , and Fastboot.exe to the folder I downloaded from htcdev "fastboot-win". I then ran cmd prompt from that folder (shift + right click) and entered the command "fastboot oem get_identifier_token" which finally worked.
TLDR: copy adb.exe, adbwinapi.dll and fastboot.exe from SDK install folder to the folder you downloaded from htc dev unlock.
hopefully this will be useful for someone stuck with my same issue.