It was bit difficult to create a server side office automation and now it is easy in VSTO 2.0.Here is nice article from MSDN.

Overview of Server Capabilities in Visual Studio 2005 Tools for Office

Introduction
Architectural Considerations when Using Server Data Capabilities
Deployment and Security Considerations
Sample Solution Development Steps
Code Security
Conclusion
Additional Resources
About the Author

Cheers..

Suresh Behera