How to install SQL Server 2008 32 bit to Windows 2003 Server 64 bit?
You can run 32 but programs on 64 bit OS normally. I'm typing this in 32 bit Opera on Windows 7 x64.
It's only where you need drivers (mostly) that you need x64 code to match
From the SQL Server MSDN Requirements, look at OS requirements for "SQL Server 2008 R2 Standard (32-bit)"
Install all your prerequirements and then run the setup file in the x86 folder. If you run the setup in the root folder it will choose the architecture for you.