OSDev.org

Strange, today, I log in, the title says 'guest', and I'm online at the bottom of the screen. Or, I log in, am logged in and from the moment I change the screen, I'm a guest. Is there another Kon-Tiki, or is it some kind of error (which I doubt).

I get that too, use this link instead.
http://www.mega-tokyo.com/forum/index.php

That is just a problem with the cookies when you visit from a different url than www.mega-tokyo.com/forum/

as long as you use that url I just wrote you shouldn't be put as guest(unless you logout).

I had this problem too when I upgraded to windows xp, I found that the solution was that internet explorer's security setting was on very high, and it wasnt saving any cookies. so I just went into internet options and lowered the security level.

Yes that too can cause it. For some reason the devs of YaBBSE decided to use cookies when making YaBBSE... glad they don't use them for YaBBSE2 though, instead it will use sessions, that will fix everybodies cookie problems.