I am wondering why would you like to do that… however, you would be moving backwards. If you are having issues with the Vista, Vista SP1 has cured most issues that existed in the initial release and that would be your quickest path to happy computing. If you are experiencing application compatibility issues, I would then test those on Windows7. If still have problems contact your vendor and upgrade the application.
If you are just totally disgusted with the product and Microsoft, then you have the choice to move to another OS. And yes, you can go back to XP SP3 which is a solid OS.