If you are not developing software, you don't need the Java Developer Kit (JDK). But you may wish to install the Java Runtime Environment (JRE). This allows you run cross-platform applications and web applets which are impossible to run without Java. Here are just a few applets: http://java.sun.com/applets.
Reformatting after a crash sounds rather severe, here is the Java download for Microsoft Windows:
http://www.java.com/en/download/download_the_latest.jsp
This forum is actually devoted to the Java Desktop System, an alternative desktop operating system which includes a Java runtime engine, an office suite, a browser and an email program.
Good luck!