Problem with path when building on Windows 7, 64 bit

[indy31]

In building my installer I have to include some files from a 32-bit library located in Program Files. Because my system is a Windows 7 64-bit it searches for the files in "C:\Program Files" instead of "C:\Program Files (X86)".

Currently we use the following code:

!define common_files "C:\Program Files\Common Files"

!insertmacro InstallLib REGDLL SHARED REBOOT_PROTECTED "${common_files}\Microsoft Shared\DAO\dao360.dll" "$COMMONFILES\Microsoft Shared\DAO\dao360.dll" "$COMMONFILES\Microsoft Shared\DAO"

The code should be changed so it works for my 64-bit OS, as well for my colleagues on 32-bit XP.
Is there a way to change the hard coded define 'common_files' to something dynamic?

I tried to use $COMMONFILES but this is not defined during compilation.