multiple JREs : Java Glossary

If you want to install multiple JREs, first uninstall all your JDKs and JREs, in order, uninstalling most recent to oldest. Manually delete all the Java directories, and delete all copies of java.exe, javac.exe, javaws.exe etc. you find anywhere on your machine including in C:\Windows\System32. For extra safety delete all the Java registry entries too. Then install the JREs and JDKs the oldest version first and install them in order. If you want the JDK, there in no need to install the corresponding JRE as well. You will get two JREs bundled with the JDK automatically.

Then instead of saying java.exe, use the fully qualified name to select which version of the java run time you want e.g. C:\Program Files\java\jre6\bin\java.exe. You can use the Java control panel to select which JRE to use in Java Web Start and in Applets, but not from the command line.