Newbier Question. Permissions needed for writing file on Server

My ASP.NET application needs to write a file to a directory on the server. The code is all written, but when I run the application, I get an access denied exception. I've tried giving read/write access to both the ASPNET user and the Internet Guest Account (IUSR_<systemname>) but I still get the access denied exception. How do I do this?

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