Why can't I login in on my computer?

Couple of weeks ago, I had a problem with startup (see here), but eventually managed to fix it.

This morning I turned my computer on, and Firefox crashed on me. I couldn’t get it to force quit, so I simply killed the power – I had no other option.

Now when I start up, though, OS X loads fine and takes me to the login window. I enter my password, the window says ‘logging in’, then disappears, the computer idles for a few seconds, and then the window reappears! This continues no matter how many times I try to login.

Help please!

edit the problem is very similar to the one described here, but I am running Tiger, and my computer is refusing to boot from my install disc…