It's not Windows but MS-DOS, which allows for pure binary executables (COM format) that are nothing but instructions. Thus even a single-byte file is executable (C3, RET - i.e. exit immediately.)

AFAIK Linux has a higher minimum size, as the ELF header alone has to be at least 45 bytes:

http://www.muppetlabs.com/~breadbox/software/tiny/teensy.htm...