Maybe it works differently for you, but on my Windows 7 system, FTB does not register as an installed program and this method doesn't work.
@asb3pe above describes how to at least remove the "modpack" folders, if not the actual FTB install files. Those are buried in your %appdata% area if you are on Windows, and it generally isn't a good idea to go puttering around in there if you don't know what you're doing.
Edit: Just browsing I notice ftblauncher files in %appdata%/local/ftblauncher and %appdata%/roaming/ftblauncher -- Unless you're planning on nuking it from orbit, I'd leave those alone and go with the workaround offered by
@asb3pe