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 & protection from security issues such as buffer overruns.
SP1 also provides support for Windows XP Service Pack 2 to provide a safer, more reliable experience for customers using Windows XP.