In SQL Server 2008, the x64 and Itanium versions of sqlncli.msi will install the 64-bit and 32-bit versions of SQL Server Native Client.  The x86 version of sqlncli.msi will only work on 32-bit Windows operating systems and will install the 32-bit version of SQL Server Native Client.  You will not be able to run the 32-bit version of sqlncli.msi on a 64-bit Windows operating system.

In prerelease versions of SQL Server 2008, you may find that the 32-bit version of sqlncli.msi does run on a 64-bit operating system.  However, this may cause unexpected results.  Use the appropriate 64-bit version of sqlncli.msi to install both the 64-bit and 32-bit version of SQL Server Native Client on your 64-bit Windows operating system.