My o key fell off on my laptop; so I wrote a program that replaces two 0 keystrokes within 200 MS of each other as an o, two 0 keystrokes within 700 MS of each other as a 0 and ignore the rest; so I could use my laptop before I get around to replacing the keyboard.
Wow; I didn’t know this would be so popular :p
As for how – Microsoft exposes a nice little API feature called Hooks.
Using that hook; I was able to write a “filter” that did what I needed it to do (hint: if you return 1 with your callback windows will not process the keystroke).
The reason I know about this actually is not because I was writing a keylogger – but because I wrote a program smiler to Synergy a while ago.
And yes. I did write another program that swapped alpha-numeric keys with a random alpha-numeric key and yes; it was really funny 😀