Ubuntu :: Can't Log In To U10.04 / Enable This?

I've been running 10.04 on a partition of my Dell laptop successfully for weeks. Now, I get the login screen, and when I give my username and password and click "Ok" it logs in, looks like X starts, then immediately goes back to the login screen with the drum sound (is that a bell?). If I type the wrong password, it doesn't log in, so I know it's getting past authentication.

I've tried logging in as one of several users.

I've tried running in recovery mode and running "clean", running "repair packages" (it downloaded and installed a lot of packages), running in safe graphics mode (failsafeX). None of those had any effect.

I've tried looking in /var/adm/messages and looking at X logs that are presented in safe graphics mode. I see tons of stuff, and none of it obviously stands out as an error.

It is likely almost out of disk (maybe 300M left?), but I can't find evidence that that is causing this problem.

I'd like to somehow repair my Ubuntu install without wiping it.