Access to Path #### denied - ASP.NET 4 Intranet Application

Hi
I manage our company intranet and it has been running fine until we got a new server installed recently.
However, the Intranet itself (an asp.net 4 application) has always run on its own Windows XP box ceonnected to the network.  It is still runnng fine but now it cannot access the shared directory on the new file server (which is where all it's files are written / read from).  The actual error says:
"ASP.NET is not authorized to access the requested resource. Consider granting access rights to the resource to the ASP.NET request identity. ASP.NET has a base process identity (typically {MACHINE}\ASPNET on IIS 5 or Network Service on IIS 6 and IIS 7, and the configured application pool identity on IIS 7.5) that is used if the application is not impersonating. If the application is impersonating via <identity impersonate="true"/>, the identity will be the anonymous user (typically IUSR_MACHINENAME) or the authe ...