Microsoft Knowledge Base Article 834647 contains the following summary:
When try to install a Windows service pack, you may receive an error message that tells you that the service pack did not install correctly. This issue may occur if you do not have the required registry permissions. This issue may also occur if there is a problem with your user profile. You can identify this issue by examining the Svcpack.log and Setupapi.log files. After you identify the issue, you can resolve it by correcting the permissions on the affected registry keys or by recreating your user profile, depending on the issue that you identify.