Trouble accessing secure websites Vista machine

Is it an exeptional network setup? Does it use a proxy or something like that? does it have an adapted hosts file, does the workstation use automatic IP and DNS resolution? Are there any settings in the router that are other than the factory settings? Does the user have the problem with ALL secure sites, or just a few? does he have it all the time, or incidentally?

So many places things can go wrong, that there really isn't anything sensible to say about it.

Flushing the DNS isn't that hard, you can let the user do this once more. See if it helps again