Giving an ASP.NET site write privileges 

IIS7 (ASP.NET 4.0)

To enable write access under IIS7, go to Web Applications in hostControl, select the Edit option and tick the ASP.NET Write checkbox.

IIS6

To enable write access to your ASP.NET site you will need to enable Impersonation in the web.config file.
The line should look something like this and be within the system.web tags.

<identity impersonate="true"/>