All in the question. Apple's installer whines I'm using the wrong one with 32bit and that I'm not using Vista or 7 with 64bit.
first make sure you have sp3. (the following worked for me on a x64 vista platform.) there are two program files folders in a x64 system. the x86 for 32-bit apps and the x64. generally you're better off with the 32-bit v and make sure it gets installed in the correct folder. also check the compatibility - right click on the itunes .exe file and select properties from the bottom of drop down list - 3rd tab should be it - select the xp w/sp3 mode. you might have to play with it here and there as there are not many apps that are coded exclusivey for a x64 environment and then you have the apple v windows issue. if possible ($$$) and hw permitting go w/win 7 for the w.
You can install the package by bypassing the unnecessary artificial restriction preventing installation in XP x64. Use 7zip to extract the files in the install package.
The complaint is from the SetupAdmin.exe file that simply invokes the msi files. You don't need to use this file, it provides one-click installation. You can install the component MSI files individually...
... except the iTunes64.msi because it is marked to restrict the file to OS versions that correspont to Vista or later. Use an MSI file editor (Microsoft's Orca MSI editor is free) to set the restriction to an OS version corresponding to XP x64 or later to allow the file to install.
For step by step instructions see the guide at; http://skunkwerks.wordpress.com/2008/04/13/getting-itunes-to-work-with-windows-xp-x64/
From a Google search it seems that some iTunes versions complain on startup that they are incorrectly installed on XP x64, and then operate correctly anyway. The message is harmless albeit annoying but reinstalling does not resolve the issue.