You do do not need to and should not uninstall Java to revert to an older version. There is a utility in your Utilities folder named Java Preferences. It will allow you to select several versions of the Java Virtual Machine.
It's much easier using an older JRE on Windows than it is in Mac OS X.
If you want to make sure you're using a specific version of Java on Windows, the easiest way will be to uninstall all of the versions you aren't using in Add/Remove Programs via Control Panel.