Exception in thread "main" java.lang.NoClassDefFoundError: sampl

Hi, I know this is a common error, but this occurs only in some of my
programs.
I run java on windows XP SP2, jdk1.5
My sampl.java file is like this:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class swing_app {
    public static void main(String[] args) {
        JDesktopPane desktop = new JDesktopPane();
        desktop.add(desktop,BorderLayout.CENTER);
        JInternalFrame internalFrame = new JInternalFrame("Internal
Frame",true,true,true,true);
        internalFrame.setBounds(50,50,200,100);
        desktop.add(internalFrame,new Integer(1));
    }
}

It compiles giving me a "sampl.class" file but when i type this


I get this error: - Exception in thread "main"
java.lang.NoClassDefFoundError: sampl

I don't get this error in a simple "Hello world" program.....
This is how i've set my paths:

Path=c:\j2sdkee1.3.1\bin;%SystemRoot%\system32;%SystemRoot%;%SystemRoot
%\system32\WBEM;%SYSTEMROOT%\system32\WBEMC:
\PROGRA~1\COMMON~1\AUTODE~1;.;C:\Java\jdk1.5.0_09\bin;c:\Java
\jdk1.5.0_09\lib;

CLASSPATH= .;C:\Java\jdk1.5.0_09\lib;C:\Java\jdk1.5.0_09\bin;C:
\j2sdkee1.3.1\bin;

Please help!!