Hola de nuevo Scho,
Parece ser que ASPNET (usuario bajo el que se ejecuta cualquier instruccion en el sistema, provinente de una aplicación ASP.NET) no tiene permisos para interactuar con el escritorio. Ahi van los pasos para darle permisos, en inglés
Código:
RESOLUTION
These are steps that needs to be followed,
1. Give permission for ASP.NET worker process account (<localmachine>\ASPNET) to interact with desktop or allow ASP.NET worker process to run in SYSTEM account.
To know how to allow worker process to run in SYSTEM account and to know the default permissions of ASPNET account, check this article
INFO: Process and Request Identity in ASP.NET (http://support.microsoft.com/default.aspx?scid=kb;en-us;317012)
2. Enable IIS Admin Service to interact with desktop
To configure this, follow this steps.
a. Open Control Panel and follow these steps:
For Windows NT: click Services.
For Windows 2000, Windows XP, and .NET Server: click Administrative Tools, and then click Services.
b. Double-click IIS Admin Service.
c. On the Log On tab, select the Allow Service to Interact with Desktop check box.
Note: This article assumes that the IIS Admin Service runs as a local system.
d. Stop and restart the IIS Admin Service.
Obtenido de
http://support.microsoft.com/default...b;en-us;555134
Saludos