If you get these annoying messages suddenly on your website, it is very well possible that some one has tampered with your php files.
Warning: Cannot send session cookie - headers already sent by (output started at c:\wwwroot\mysite\config.php:9) in c:\wwwroot\mysite\config.php on line 11
Solution: Although this is not a 100% solution but this worked for me. Open the file mentioned in the error message (config.php in my case) and go to the end of the php closing tag. If you notice extra lines after the closing tag delete it. Please traverse all the nested include files and fix these extra lines. You should be good to go.