followFollow the steps below
1.Go to your IIS manager (type inetmgr in Run hit Enter)
2.Select and open the SharePoint web application in which you have this error
3.Double click on Authentication icon. here you will see different types of authentication listed for your Web Application:
4.In that list Right click on Anonymous Authentication and Enable it if its disabled
Go to your IIS manager (type inetmgr in Run hit Enter)
Select and open the SharePoint web application in which you have this error
Double click on Authentication icon. here you will see different types of authentication listed for your Web Application
In that list Right click on Anonymous Authentication and Enable it if its disabled
nowNow try opening site in SharePoint designer again
forFor more details you can check here : http://support.microsoft.com/en-us/kb/2758431