The process under which you web application is running does not have write access to the directory. That is the default setting for default account under which asp.net run. You options are - either you can use a more privileged user account (which isn't a very good idea) or give write permission to that folder for that a/c. You can find ways to do that here -
https://www.google.co.in/#q=file+upload+permission+asp.net[
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On more searching you can find an article on MSDN explaining this in detail.
Hope that helps!