I suggest you unintall you JDK1.3 and
delete the remnant directory. Restart your machine. Install again. Install JDK to the directory such as C:\jdk1.3. Do not change it to other directory.

Your probiem is a classpath problem.
To check your current classpath, you can type
echo %CLASSPATH%.

It is good to use BAT file. To set PATH, you say set PATH=C:\jdk1.3\bin; %PATH%.
To set CLASSPATH, you say
set CLASSPATH=.; %CLASSPATH%.
Remider: JDK1.2 and JDK1.3 can find System classes such java.lang.String automatically.

I guess you problem is as follows:
You put your Hello.java and Hello.class
in some directory such as C:\jdk1.3\bin or
C:\myprojects. If you run your Java example at C:\, definitely your JVM cannot find Hello.class. Reminder again: "." means the current directory.


Jbuilkder has a closed envirenment and it does not interfere with JDK1.3,

Jdk1.3 does not contain j2ee.jar. You don't need j2ee at present because you are a beginner.