Still looking for a solution to the "Could not find the main class" error.

I've been trying for a few days to find a solution for this but I still can't get it to work. I'm running 32 bit Windows Vista and I've downloaded several versions of java runtime but all of them produce the error "Could not find the main class. Program will exit." whenever I try to launch the .jar file. I've seen other posts on here about this error but none of them have clear answers that work. Has anybody found a solution to this yet?