Raymond Chen: Commenter Koro asks why you can rename a COM file to EXE without any apparent ill effects. (James MAstros asked a similar question, though there are additional issues in James' question which I will take up at a later date.)
Initially, the only programs that existed were COM files. The format of a COM file is... um, none. There is no format. A COM file is just a memory image. This "format" was inherited from CP/M. To load a COM file, the program loader merely sucked the file into memory unchanged and then jumped to the first byte. No fixups, no checksum, nothing. Just load and go.
The COM file format had many problems, among which was that programs could not be bigger than about 64KB. To address these limitations, the EXE file format was introduced. The header of an EXE file begins with the magic letters "MZ" and continues with other information that the program loader uses to load the program into memory and prepare it for execution.
And there things lay, with COM files being "raw memory images" and EXE files being "structured", and the distinction was rigidly maintained. If you renamed an EXE file to COM, the operating system would try to execute the header as if it were machine code (which didn't get you very far), and conversely if you renamed a COM file to EXE, the program loader would reject it because the magic MZ header was missing.
Windows, File Extensions, Knowledgebase