Unable to install Service Pack 1 for SQL Server 2005 version.

SQL Server 2005 has many key new features including a production-ready version of database mirroring from Service Pack1 installation, in which the primary production server is mirrored at all times by a standby server. "This allows for automated, seamless failover between primary and standby server, if the primary server needs to come down".  SP1 also includes the new SQL Server Management Studio Express. This express management tool offers management dialogs, graphical designers for databases, tables, views and queries, and graphical query execution plan display, all delivered in user interfaces that are now seamless across SQL Server management tool offerings.

if SQL Server 2005 Service Pack 1  fails to install, the first check to perform is whether the hardware has met the requirements such as : 

• Supported Operating Systems: Windows 2000 Service Pack 4; Windows Server 2003 Service Pack 1; Windows XP Service Pack 2

• 32-bit systems (x86)
  • PC with Intel or compatible Pentium III 600 MHz or higher processor (1 GHz or faster recommended)
• 64-bit systems (x64, ia64)
  • 1GHz or faster processor
• Minimum of 512 MB of RAM (1 GB or more recommended)
• 675 MB of available hard disk space 

The login used to install SP1 must have Administrator rights on the server that includes on Operating system & SQL Server.

Make sure you are using correct version and file of Service Pack 1, for instance you will not be able  to apply [SQLServer2005SP1-KB913090-ia64-ENU.exe] file to a 32-bit SQL Server.

I have also encountered another error which is different to the above errors and other generic errors, which is "Unable to install Windows Installer MSP file".

Workaround:

Ensure the login that is used to install Service Pack has a Grant of FULL Control permission on Data folder of SQL Server, the referred registery key is:

%ProgramFiles%\Microsoft SQL Server\MSSQL.X\MSSQL

Lastly, when I have approved MS PSS, they have provided the fix by referring few tweaks on registry keys and explanation for one of the root cause as "SQL Server Setup Support files that are installed on the computer by one of the previously listed products are not compatible with the SQL Server Setup Support files that are included in SQL Server 2005 SP1."

The steps to tweak the registry key has been documented under the Knowledge Base Article (KBA) [KB918357]  :

 http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B918357