Check if a URL exists

http://stuff.seans.com/2009/01/05/using-httpwebrequest-for-asynchronous-downloads/

http://stackoverflow.com/questions/924679/c-how-can-i-check-if-a-url-exists-is-valid

http://blogs.microsoft.co.il/blogs/dorr/archive/2008/09/02/how-to-check-if-a-file-exists-over-http.aspx

http://www.developerfusion.com/code/4654/asynchronous-httpwebrequest/