Exchange 2010 CAS installation Error

get following error, when installing of exchange 2010 in exchange 2007 environment.

CAS: 

The following error was generated when "$error.Clear(); 
          $InternalOwaUrl="https://" + $RoleFqdnOrName + "/owa";
          new-OwaVirtualDirectory -DomainController $RoleDomainController -InternalUrl $InternalOwaUrl;
        " was run: "Web object 'Exchange' already exists.".

please advice

AD # windows 2008 R2

pls advice.