Leia a variável de ambiente PATHEXT artigo:
PathExtenvironment variable returns a list of the file extensions that the operating system considers to be executable. When executing a command line that does not contain an extension, the command interpreter (cmd.exe) uses the value of this environment variable to determine which extensions to look for and in what order.
Para restaurar variável (s) quebrada (s), leia e siga Como definir o caminho e as variáveis de ambiente no Windows