Hi stef,

From my understanding to your issue, you can handle specific excel event to log the user activitiy. If so, there is one KB article introducing related
information for you. Please go to:
302815 HOW TO: Handle Events for Excel by Using Visual C# .NET
http://support.microsoft.com/?id=302815

Please feel free to let me know if you have any further questions.

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Wei-Dong Xu
Microsoft Product Support Services
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