yes you can, but you are only proving usability for the users skill set that you test with.
you can get _very_ different answers to usability questions from users that have never seen a computer before (but you can't repeat the test with those users, because they now have the experiance of your first test) then if you test with people who have used windows for years then if you test with people who have used Macs for years.
which set of results is 'more valid' is definantly arguable.
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