Can't run java programs

I'm having trouble getting java programs to run on my computer. I have no problem compiling programs but whenever i try to run them through the command prompt window i get the following error:
Exception in thread "main" java.lang.NoClassDefFoundError: hello
where 'hello' is the name of the 'hello world' test program i made (just prints the words 'hello world' world on screen.)
The programs run on eclipse but i really prefer using the good old command prompt window. Can anyone explain to me what's wrong here (I'm guessing it's some sort of windows/java sdk setting that must be off.) Thanks in advance!!!

I thought that this was resolved a couple days ago but apparently it wasn't (it was the temporary fix that was working.)
I still get the same noclasseffounderror whenever i try to run the java program. The link tells me to add the correct 'path' to the system folder to make it work. It's actually already there (otherwise the javac command wouldn't work either.) However, the java programs do run whenever i execute the command
set classpath=
However, i have to do this every single time i open up a new shell window. Anybody know what the problem could be given that it works when i 'clobber it into working' as the tutorial put it?

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