Since the process starts with the system directory, things quickly go bad. Once Drwtsn32.exe has been replaced by Scar-H, it gets called recursively, resulting in a freeze.
And then trying to reboot the system fails, as critical system files have been replaced.
The worm spreads by installing AutoRun files on devices mapped to drive letters.
Cleaning up involves removing all the copies of Scar-H and its associated registry entries, and replacing the real files from a backup or by reinstalling the relevant software.