How do you impersonate an Active Directory user in Powershell?

I’m trying to run powershell commands through a web interface (ASP.NET/C#) in order to create mailboxes/etc on Exchange 2007. When I run the page using Visual Studio (Cassini), the page loads up correctly. However, when I run it on IIS (v5.1), I get the error “unknown user name or bad password”. The biggest problem that I noticed was that Powershell was logged in as ASPNET instead of my Active Directory Account. How do I force my Powershell session to be authenticated with another Active Directory Account?