Ok this is easy enough to fix...
The way the installer looks for the JDK is hard coded to look in the registry for version 1.4 only.
Fix that to look for any version, preferibly the latest and everything should work again.
I found this out by looking at the zip version of the tomcat add-on.
Changing "install_java.bat" to look at 1.5 instead of 1.4 made the wole lot work