To check which version of Java is installed, go to Start, choose Control Panel, then click on Java.
In the Java Control Panel, click on About…

Java Control Panel

This will display which version of Java is installed.

About Java window

If you need to uninstall Java 7, Update 11 and install Java 6, follow these instructions.

Step 1. To uninstall Java 7, Update 11, go to Start and choose Control Panel. In Control Panel go to Programs and Features. Find Java (there may be more than one), highlight and choose uninstall.

Programs and Features window

Step 2. Install Java 6 by going to this link: http://java.com/en/download/manual_v6.jsp.