You can have both the Sun JVM and the Microsoft JVM installed at once with no conflicts. You can also have multiple versions of the Sun JVM installed at once (I use 1.4.0_01 for just about everything at work, but there's one application which requires a particular 1.3.x release and nothing else). Your browser will use one of them as the default for applets, but applications will still be able to pick a particular one if required.