Sd 5.1 has 2 types of clients. Full client which requires you to install it manually and comes with it's own java lib and the java web start client which requires you to preinstall java 1.4.2 on your PC. I suspect you are using the web start version and missing the java 1.4.2.
I have been told that the sd 5.1 web start client does not work with java 1.5 and java 1.6. The sd 5.1 full client should be fine as it has it own self contained java lib but not the sd 5.1 java web start client as that uses the OS's java.