Startup disk full error - boots normally but won't start finder

I'm having a problem in which my iMac when it boots gives me an error that the startup disk is full, but other than that it can boot normally and allow me to log in, but after that the startup stalls. I never get past the default "outer space" desktop image and finder never starts, so there's no icons, no dock, and nothing on the menu bar. What's weird is that Safari and Mail start and I can see them because I believe I have them starting automatically at boot. Safari works (I'm using it now to enter this discussion). Mail on the other hand starts but is hung checking for the servers for incoming mail.

I've tried to boot in safe mode (holding down Shift key at startup), but it doesn't work.

I've tried to boot in single user mode (holding down cmd-S at startup), but it doesn't work either.

I've tried to boot from Snow Leopard installation media (holding down C at startup), but it doesn't work as well.

Each time I try one of these alternate boot methods, it just takes me to the normal login screen with the startup disk full message. I click OK, login, and get stuck.

I'm just trying to get the finder started so that I can delete some files, but I just can't get that far. It's strange that the browser works perfectly and internet connection is fine, but nothing else works.

Thoughts? Thanks. - Mike