The primary focus of Microsoft .NET Framework 1.1 Service Pack 1 (SP1) is improved security. In addition, the service pack includes roll-ups of all reported customer issues found after the release of the Microsoft .NET Framework 1.1. Of particular note, SP1 provides better support for consuming WSDL documents, Data Execution prevention and protection from security issues such as buffer overruns.
Download: .NET Framework 1.1 Service Pack 1
Download: .NET Framework 1.1 Service Pack 1 for Windows Server 2003
More Information
KB 867460: List of bugs that are fixed in the .NET Framework 1.1 Service Pack 1 (SP1)
KB 318785: Determine wheather service packs are installed on the .NET Framework