NoClassDefFound Exception

It seems that any class I try to run now compiles fine and then throws a NoClassDefFound error when I try to run it. I've unset the classpath (I'm on OS X) and it still does this. I've been playing with Eclipse as well, and it was working fine, although recently I updated it pretty heavily; it is possible that that update was around the right time to have caused this issue. I've thought about reinstalling the jdk, but I don't know how to do this in OS X, and I'm enough of a Java newb that I don't know if that's the right thing to do anyway.

Help!

[573 byte] By [dwingrahama] at [2007-11-27 9:20:00]