The Microsoft Java VM available on older releases of Windows is not at the required level, you will need to get a new one.

Java VMs for Windows, Linux and Solaris are available free from Sun Microsystems (the inventor of Java). If you just plan to run the compiled classes and you are happy to install the Sun Java VM as your default, then get the J2SE v1.4.2 JRE. If you plan to make changes to the code or you do not wish to install a default Java VM then get the J2SE v1.4.2 SDK. When you run the SDK installation you are given the choice of installing a default VM or not. If you decline then the SDK VM is available to you like any normal application (eg on Windows you might invoke it C:\MyApps\sdk\bin\java.exe), but there is no attempt to make the VM available by default.

An additional advantage of the Sun SDK is that it supports the 'server' option which results in higher performance for processor intensive applications like the IUF Loader.

Java VMs for other platforms are available from the platform vendor.