OS password accidental "reset" Help...

If you did something stupid, like right-clicking the Home directory to change the short name , and the OS password isn't working anymore because obviously you're not supposed to change the short name, what do you do? (I assume all my settings and preferences, along with the password obviously, were reset to some default setting)

Especially, what do you do if you can't locate the OS disc very easily?

(Yeah yeah, I should have known that it wouldn't be as easy as that... but I'll never do that again )

If it makes any difference, I'm the only user, and I haven't changed any OS settings out of the ordinary.