since I have 32 bit version of SQL Server Evaluation Edition. Download the required edition as per the SQL Server 64/32 bit or OS what you have.
If you have SQL Server RTM Edition 32 bit you will have built Number as Microsoft SQL Server 2008 (RTM) - 10.0.1600.22 (Intel X86).
After installation of SP1 the build number (as on 22 May 2009) will be
Server 2008 (RTM) - 10.00.2531.00 (Intel X86).
Installation process are as follows
Step 1 : Download SP1
Step 2 : Install the SP1. Double click on the downloaded file
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Summary :
There is nothing complex in this installation. After the installation you need to restart the computer to affect the installation