Exception in main.....Osv

Hvorfor for jeg denne fejl:
Exception in thread \"main\" java.lang.NoClassDefFoundError: Server
-----------------------------------------Kode

package Clientserver;

import java.io.*;
import java.net.*;
import java.util.*;
import java.lang.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Server
{
  public static void main(String args[])
  {
    ServerSocket ser=null;
    Socket soc=null;
    String nyString;
    String str=null;
    try
    {
      ser=new ServerSocket(1003);
      soc=ser.accept();
      InputStream o=soc.getInputStream();
      ObjectInput s=new ObjectInputStream(o);
      str=(String) s.readObject();

      OutputStream op=soc.getOutputStream();
      ObjectOutput oo=new ObjectOutputStream(op);
      System.out.println(str);
      oo.writeObject(str);
      oo.flush();
      oo.close();
    }
    catch(Exception e)
    {
      System.out.println(e.getMessage());
      System.out.println("ERROR DURIN SERIALIZATION");
      System.exit(1);
    }
  }
}

------------------Er det min classpath?
Hvordan skal den sættes...Jeg kører win2000pro

Mvh Casualty