ArcGIS is a 32-bit process, and you want to ensure your Add-in is 32-bit (x86) even when run on a 64-bit OS. And my understanding is that the "Any CPU" setting gives the .net JIT compiler some leeway on what it outputs depending on the host OS.
I'm not sure that this is your problem, but it is the first thing I would check. The problem you describe happens when ArcGIS cannot load the Add-in assembly even though it can read the config.esriaddinx and create toolbars and buttons.
Have a look at these two forum threads:
Make sure the namespace in your code matches the one in the Config.esriaddinx file.
If none of that helps, maybe there is an issue with XP 64-bit. Does the ESRI MapCenter Add-In that is installed with ArcGIS 10.0 work? Select the ArcGIS Online... item on ArcMap's File menu to test it.