Was just burning (with Windows7 included image burner) some backup ISO images of Windows Vista x64 final (MVPs who had submitted bugs on Microsoft Connect during Vista beta test cycle were getting both x86 and x64 final Ultimate versions for free). After reinserting the disk I noticed it was trying to run sperr32.exe instead of setup.exe.
Looking at autorun.inf it was obvious that the reason was the x86 version of Windows7 (expired RC in fact) I had on that Intel EMT64 (AMD x64 compatible) machine. Vista x64 autorun.inf had special section that would launch setup.exe (for in place WindowsXP to Vista upgrade I guess) on a x64 OS and sperr32.exe with param x64 passed to it.
The sperr32.exe executable seems to check the OS architecture and if it’s not same as the param passed to it (assume valid params are x86, x64 and maybe IA64 too for Itanium) just show the message "Your computer cannot run this version of the Operating System" at an "always on top" (to not go by accident to the background without the user noticing) modal dialog box. Passing invalid params there like x65 etc. just does nothing.