Check Java on Windows XP/Vista

Go to your command window and run

java -version

This should return something like

java version "1.6.0_03"
Java(TM) SE Runtime Environment (build 1.6.0_03-b05)
Java HotSpot(TM) Client VM (build 1.6.0_03-b05, mixed mode)

If the build number is less than 1.6.0 you will need to install a later version of Java. If it returns

Java is not recognized as an internal or external command,operable program or batch file.

This means that either it is not installed or you havent set up your paths properly, check by looking at the installed program list.