First, make sure your computer contains a Java Run-Time Environment, version 1.3.1 or later. To test which Java version is presently installed on your computer, type "java -version" at the DOS/shell level. To download a suitable JRE, visit http://www.javasoft.com/
Second, if your computer contains a JDK (Java Development Kit), make sure that the "bin" directory of the JDK (usually something like JDK...\bin) is included in the PATH system variable on your computer. Also, make sure that the CLASSPATH system variable contains the tools.jar file, which is located in the "lib" directory of the JDK.
Changing system variables in Windows is done by invoking the Control Panel and updating the variable line in System->Advanced->Environment Variables. In older versions of Windows, up to and including Win98, it is done in the "set <variable-name>=..." line in the c:\autoexec.bat file.