File Format Fundamentals: OPC
Both Office and XPS uses OPC (Open Packaging Conventions) to have a XML inside of .ZIP file format. I’ve been very excited about OPC for a number of uses since we first shipped support in .NET 3 with System.IO.Packaging apis.
The Windows team continues to innovate in this area in Windows 7 and beyond and now they are blogging about the work they are doing: http://blogs.msdn.com/opc/archive/2009/05/18/adventures-in-packaging-episode-1.aspx