Trouble accessing secure websites Vista machine

Have a client that keeps having a problem accessing secure websites. Have followed the instructions from Microsoft, have reset router and so on.

This last time when we thought we had it fixed, I flushed the DNS. this lasted about a week and tonight I get an email saying its happening again.

Machine has been scanned multiple times for bugs, all up dates done.

Even tried Firefox, same problem.

Any thoughts?

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Thanks,
Digitalshooter
PS: all posts are just my opinion!