Intermittant trouble automating Word/Excel from download

Windows Vista Business SP1
IE8
Office 2007
Windows Defender - disabled.

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I have a web application that the customer downloads files as either .pdf,.doc,.xls. The files are retrieved through anASPX file. This methodology has been in production for 5 years without a single report of trouble.

The download occurs as a content-disposition of "inline", with content-type of either application/msword, application/vnd.ms-excel, or application/pdf.

The user has demonstrated (to me via webex) an intermittant problem when automating the .doc and .xls files in word/excel. The problem manifests itself as an errormessage of:

Error processing request:Object reference not set to an instance of an object

This errormessage is located in a new browser window, where i would have expected the content to appear.

If the user clicks the link 3 times, it is likely that one of those times will successfully open the file in either Word or excell as appropriate.

The files can be "saved as", and then opened without any problem. This works 100% of the time.

Opening a PDF file works 100% of the time.

The error only occurs when a non-adminstrator user is logged in.

Configuring the browser to trust the site did not have an effect on the problem.

Does anyone know what is wrong?