Login problems under kubuntu

I've taken my first foray from windows a couple of months ago and installed kubuntu on my PC at home. I've set up two users and found the whole thing rather straightforward and easy. I've updated and installed things using the adept manager, set up my Mp3's and started downloading using ktorrent.All uneventful until a couple of days ago, when I restarted the system, and could no longer log in as either of the two users I'd set up.After supplying the correct password the screen goes black for a few moments then just returns you to the login screen.Strangely, I can still login under command prompt (although I'm in slightly over my head there), and also I can log in as root, which I understand is disabled by default, and I haven't taken any steps to enable it.I can't think of anything I've done to bring about this behaviour, I hadn't done an update for a while and I had just left the PC on to play music..Anyone got any ideas?