I concur that older drivers shan't work and that there is no substantial difference betwixt Windows 5.0 and 5.1 in this regard. They have simply discontinued official support to reduce costs (and/or Microsoft no longer certifies).
I would have thought maybe there was a mistake with the Asus disc if the same 64-bit error did not occur with the AMD download. But I guess the "incompatability" in the driver setup just manifests oddly.
It's likely that a very simple edit to the setup is necessary -I do this sometimes to add my AGP card to the main Catalyst release rather than waiting for the inevitable hotfix driver. I don't know what that would be precisely but it should be simple enough to find at rage3d or asus support forums I guess. I would not recommend using a pre-modified setup having a mess of other alterations.
Mayhaps try this compatabilitiy patcher at rage3d