Download the Windows (Offline Installation) version of Java from
http://java.com/en/download/windows_xpi.jsp
start the installation as usual. Accept the licence agreement and then look for the following files at
C:\Documents and Settings\ <Your User name will be here> \Local Settings\Application Data\{3248F0A6-6813-11D6-A77B-00B0D0150050} (these numbers and letters are purely random so will be anything).
Note: The Application Data folder is hidden unless you use Control Panel-->Folder Options-->View-->Show hidden folders and files)
In this folder will be two files called 1033.MST and J2SE Runtime Environment 5.0 Update 5.msi
Save these files to another folder in your unattended setup. Start the installation using
c:\folder name\J2SE Runtime Environment 5.0 Update 5.msi /qb
