You need to read about Impersonation through ASP.NET! The long and the short of it is, you will need to add this to your web.config:

<identity impersonate="true"/>

And the MSDN Article to explain WHY you need to do that:
http://msdn2.microsoft.com/en-us/library/xh507fc5(VS.71).aspx

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Read this if you want to know how to get a correct reply for your question:
http://www.catb.org/~esr/faqs/smart-questions.html
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Technical Editor for:
Professional Search Engine Optimization with ASP.NET

Professional IIS 7 and ASP.NET Integrated Programming

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