I agree with not running Office on a server. Not that I have any choice in the matter :)
One thing to keep in mind with the taskkill option, is that unless you specifically plan for it (aka - singleton), you may have multiple copies of Excel (or any other Office app) running, and unintentionally close the wrong instance.
Also note that per http://support.microsoft.com/kb/257757
Microsoft does not currently
recommend, and does not support,
Automation of Microsoft Office
applications from any unattended,
non-interactive client application or
component (including ASP, ASP.NET,
DCOM, and NT Services), because Office
may exhibit unstable behavior and/or
deadlock when Office is run in this
environment.
As an alternative, there is a product called Aspose Cells that offers a product that is designed to allow you to programmatically work with an Excel sheet in a server environment. As a disclaimer, I have never personally used this product, but I have heard about it from several people I worked with in the past.
