All 64-bit Intel instruction-set architectures
uses for PCs are presently compatible with x86 (32-bit), but not with each other. On Windows, 32-bit applications are executed under WoW64 subsystem: http://en.wikipedia.org/wiki/WoW64
A separate installer (MSI) is required for each instruction-set architecture, and 32-bit installers are compatible with all modern Windows systems.
So, why do you have this weird message. I don't know exactly, would need more detailed information, but I think the key is "viewer
software". It might be not a general-purpose application, but maybe some applet of plug-in for some application or sub-system which only comes in 64-bit architecture. You should know better what is that. If this is so, you still need to make a "native" installer. By the way, why not?