Don't comment bad code - rewrite it.
The quote belongs to Brian W. Kernigham and P.J. Plaugher (and I'm sure to many others). It might sound simple... yet I perceive it as so powerful! It puts your back to the wall with the choice to devote yourself to leave your campfire slightly better than you found it... or to simply turn your head.
Uncle Bob has a similar opinion on the subject:
"Ooh, I'd better comment that!" No! You'd better clean it!
Commenting bad code derives from the fear to actually modify it. But beware:
Fear is the path to the Dark Side.