The most reliable solution would be to create a 32bit loader application that detects the architecture and then downloads and installs the appropriate version of your application.
I've checked the other two answers from RC and Pino. They both do not work because of the same problem as you suggest - 32-bit IE on 64-bit Windows will wrongly identify the platform as 32-bit. As most people run 32-bit IE on 64-bit Windows (many plugins e.g. Flash are not available in 64-bit), there will be a lot of innacurate identifications
Lee