Here is some information on installing WSUS SP2 on a Windows Server 2008 R2 machine.
Make sure you install the following Role Services in IIS from the Server Manager:
- Common HTTP Features (including Static Content)
- ASP.NET, ISAPI Extensions, and ISAPI Features (under Application Development)
- Windows Authentication (under Security)
- IIS Metabase Compatibility (under Management Tools, expand IIS 6 Management Compatibility)
For more info check out the Microsoft Windows Server Update Services 3.0 SP2 Deployment Guide.