New-ExchangeCertificate : Cannot overwrite the output file

Error on running New-ExchangeCertificate
problem also reported here
http://social.technet.microsoft.com/Forums/en-US/exchangesvrsecuremessaging/thread/716326a3-112d-4ef7-a630-20b4d0fb7864
New-ExchangeCertificate : Either you cannot overwrite the output file c:\cert\m
ail_xxxx_com_au.csr because it is set to read-only or you have insufficient
permissions to create this certificate request.
At line:1 char:24
+ New-ExchangeCertificate <<<< -GenerateRequest -Path c:\cert\mail_xxxxx_com
_au.csr -KeySize 2048 -SubjectName "c=AU, s=Queensland, l=Oxley, o=xxxxx Pty
Ltd, cn=mail.xxxxx.com.au" -DomainName server-xxxxx, sites, autodiscover.xxxxx.com.au -PrivateKeyExportable $True
problem also reported here
http://social.technet.microsoft.com/Forums/en-US/exchangesvrsecuremessaging/thread/716326a3-112d-4ef7-a630-20b4d0fb7864
Also, even if you are logged on to the machine as an admin, you have to right click Exchange Management Shell and select Run as Administrator. Once you do this you can run the new-exchangecertificate cmdlet as needed.