In general there shouldn't be a problem with running multiple versions of the Java VM on a single machine. You just have to be careful how you set up the environment. If you are using multiple version,
you should probably explicitly set the JAVA_HOME and CLASSPATH and path to "java" and "javac" rather than rely on the environment settings.
Either set the values in batch files that will compile and execute your programs (different batch files for each version), or use a tool such as "ant" to manage your builds.
If you don't know what version(s) of Java are installed, doing "java ..." can be dangerous. To ensure you execute a specific version, do "C:\j2sdk1.4\jre\bin\java" or "C:\j2sdk1.4\bin\java" and give the classpath you want to use. You shouldn't have registry problems as long as you set up the batch file correctly.
[ October 10, 2003: Message edited by: Wayne L Johnson ]