Take a look in the following articles of Microsoft.
They're might be helpful :

How to configure Office applications for automation from a COM+/MTS package
http://support.microsoft.com/kb/288368/en-us

How to configure Office applications to run under a specific user account
http://support.microsoft.com/kb/288367/en-us

How to configure Office applications to run under the interactive user
account
http://support.microsoft.com/kb/288366/en-us

Considerations for server-side Automation of Office
http://support.microsoft.com/kb/257757/en-us

Stanos