Validation iof user password...

[jscrunch]

Hi friends,
I'd like to add certain rules for the users paswords. I e.: When they will go to change their passwords, I'd like to put an internal control to avoid them to put the username and "similars strings " as that password...
How could I do it?

I've seen that password change screen is in Self-Service user preferences...
So I don't exactly know where to locate the code that validates the password...(i suppose there is where I have to write my vlaidation code...)

Thanks,
Jose L.