KBA:The service pack update or hotfix installation stops unexpectedly when you try to install either Microsoft SQL Server 2005 Service Pack 2 or a hotfix for SQL Server 2005 SP2
Source: Knowledge Base
Product: Microsoft SQL Server 2005 Enterprise Edition, SQL Server 2005 Standard Edition & SQL Server 2005 Enterprise X64Edition
Notification Contents: New and Major Modifications
The service pack update or hotfix installation stops unexpectedly when you try to install either Microsoft SQL Server 2005 Service Pack 2 or a hotfix for SQL Server 2005 SP2
http://support.microsoft.com/kb/936488/en-US