I've done some forensic analysis on this (going back to my days in software
testing). I had an account called "New Account". I renamed it "Mike" some time ago. I then tried (and failed) to install Java 7 on the renamed account. I then created a new account (co-incidentally also called "New Account") and Java installed there just fine.
I wonder if, deep down in the bowels of my system, something is still pointing to "New Account", instead of "Mike"? And Java, when it encountered this, tried to send a message to an account that no longer existed as it was renamed?
Something to consider, anyway. Perhaps one day someone, somewhere will show up here with the same problem. If I had the time (and inclination) I'd test various scenarios to see what happens.
Anyway, thanks to all of you for trying to help me!
Cheers, Mike.