He is running :-Windows XP SP1, Office 2003, IE version 6, McAfee Viruscan + Anti spyware Module 8.0.0, Java runtime Ver: 1.5.0_10.
To give some more background, I have installed the latest JVM, and reinstalled the VF client on the users machine.
That's almost bound to be the issue - I believe you need to use an "older" version of the JVM - check the manuals, release notes, and Support KB articles for more information.