I knew there was a catch (22?) with those "teach yourself anything in 21 days" courses
Likely, the catch is you must have 21 completely free days. In addition, you must already have pre-existing skills which will allow you to pick up whatever it is you are studying (ie, for C++ in 21 days, you better already have a good programming base behind you, and be familiar with at least some of the concepts of OOP). Finally, you probably should have really good reading comprehension and retention skills - along with a strong desire to succeed.
It can be done, I'm sure (for an interesting story of "learning how to program in a short period of time", look up the story of how Sierra Online - now Sierra Entertainment - was started).
