exchang server 2010 setup

hi

i face the following error in installing Microsoft exchange server 2010:

Error:
The following error was generated when "$error.Clear();
          $vdirName = "PowerShell";
          $InternalPowerShellUrl="http://" + $RoleFqdnOrName + "/powershell";
          new-PowerShellVirtualDirectory $vdirName -InternalUrl $InternalPowerShellUrl -DomainController $RoleDomainController -BasicAuthentication:$false -WindowsAuthentication:$false -RequireSSL:$false;
        " was run: "The virtual directory 'PowerShell' already exists under 'WIN-CLIENT.test.local/Default Web Site'.
Parameter name: VirtualDirectoryName".

The virtual directory 'PowerShell' already exists under 'WIN-CLIENT.test.local/Default Web Site'.
Parameter name: VirtualDirectoryName"

what is it for, and how to solve this problem. thanks in advance.