Okay, then I am less than 90% sure now. Did you uninstall Java 7 before installing Java 6?

Well, I didn't at first and it didn't work, but then I saw this post and tried uninstalling Java 7 and it still didn't work, so...

Also, I'm 90% sure that the crash a lot of people are experiencing when changing floors is caused by Java 7. I had that problem when I first started playing. It worked fine on Linux (which has Java 6 installed), but would always crash for me on Windows. I got fed up with Java 7 when I couldn't play most of the Ludum Dare games. I downgraded to Java 6 and no longer had that problem. Ask the people experiencing this what version of Java they're using.

Nope, tried this and it (unfortunately) didn't work. I'm still having the crashing problem when I change floors.