Almost all of the characters typically used in MapComp input files are simple ASCII and therefore can be input using most editors, including those used in Microsoft Windows, such as Notepad.
The only exceptional character is the degree (°) symbol which can be used in latitudes and longitudes. The PC-8 character code for this character is 248 (decimal), 0xF8 (hexadecimal). It can be input to MS-DOS editors by pressing and holding the Alt key, typing 2, 4, 8 on the numeric keypad then releasing the Alt key.
If you do this in a Windows editor, Windows will automatically translate that character code to its own representation of the degree symbol (which is 176 (decimal), 0xB0 (hex)). Therefore, in a Windows editor, you need to use Alt 0, 2, 4, 8 (that is, add a leading zero) to tell Windows that you really want that particular numeric code. The Windows editor will display this as a lower case O slash (ø) but it will keep MapComp happy.
Alternatively, you can avoid use of the degree symbol completely and use lower case letter 'd' in its place.