Just out of curiosity - *why* are you having everyone login to comment? WordPress is already set up (in your options page) so that first-time commenters have to be approved to comment. people are *required* to leave a name, email address and comment - but it won't show up if they're "new". You moderate and decide if you want the person to be able to leave a comment or not, and WordPress learns from that. From then on, whatever you decided is what happens with the comments - if you allow them to leave comments, then they can (email and name are still required, though). If you don't, they can't. It's already built in that way.
And yes, what moshu said - if people are logging in just to be able to post as well, then they *should* be familiar with how to do it.
So I guess my curiosity is piqued, as well - why would you want people to log in to leave a comment in the first place? You say you don't have a choice, but you don't say why. If it's just to reduce spam and that sort of thing, that's what moderating first-time commenting is for, and using plugins like Akismet and Bad Behavior.
Otherwise, it may help to know *why* you're adamant about having people log in to leave a comment, because a better solution could be proposed.