Why are you using Safari 5.1? It has not been update for YEARS!
Also, why Windows 7? It has not been updated for a while, either.
When machines do not have security updates, they might not recognise a CA/cert.
However, GoGetSSL certificates should work on Windows 7 (SP1), I have tested it.
We can’t help you very much if you do not provide a domain or username.