"SP2 is an update to Windows Server 2008 and Windows Vista that incorporates improvements discovered through automated feedback, as well as updates that have been delivered since SP1. By providing these fixes integrated into a single service pack for both client and server, Microsoft provides a single high-quality update that minimizes deployment and testing complexity for customers.
Service Pack 1 is a prerequisite for installing Service Pack 2. Please make sure that your system is running Service Pack 1 before you install Service Pack 2. " -- quote from the above site.