A program?

That's a built-in feature in Microsoft Office, and works in most Office programs (but not in IE).

If you hold down Ctl while you hit the first character, the second character overprints to make a "composite." It's not a general "overstrike" thing, since lots of Ctl-x key combinations are shortcuts that will "do something" instead of setting up the combination character. Ctl-c, as an example would "copy" something, but Ctl-/ c gets ¢.

In Word, the sequence Ctl-F9 eq \o(a,b,c) F9 Ctl-Shift-F9 will print a, b, and c all in the same character space, and is a general "any overstrike" capability. Ctl-F9 inserts a "Field," the eq specifies an "equation field," \o makes it an overstrike, any number of characters, separated by commas, can be entered in the parenthesese, F9 "evaluates" the field and Ctl-Shift-F9 unlinks it to make it a "printed" object. (Note that if more than a couple of characters are overprinted, usually one of them is about all that will show legibly.)

In recent Word versions, you can also type a Unicode HEX character number and then hit Alt-X and it will be converted to the character. This is less useful than it might be though, since the character has to be present as a glyph in the font you have set when you do it. The "characters" you get this way usually are like what you get with Office's "Insert Symbol" and if you change the font after the character is inserted it often changes to garbage (and can't be changed back). Windows default installations only include a couple of fonts with reasonably complete "European language" extended Unicode character glyphs (and I don't recall which ones they are at the moment). Several fonts have a few "random extra characters" that may work, but there's little consistency between common fonts.

John