Update: These instructions still work when installing the recently released Zune 2.5 software.
Microsoft currently does not support Zune software being installed on Windows XP Professional x64. The only 64-bit operating system the software officially supports is Vista 64-bit edition. However there is a hack to get the Zune 2 software to work with Windows XP Pro x64 and the syncing with the Zune device works as well. The instructions for doing this are available on this blog post.
Here is a copy of the instructions which appear on the blog site (without the screenshots):
1. Start by unplugging your Zune from the PC running Windows XP x64
2. Remove any attempts of installing Zune in “Add or Remove Programs” in Control Panel. The Zune entry may not exist in your list, if so, skip this step.
4. Inside the extracted folder open up “packages” then double click “Zune-x64.msi”. This should bring up Windows Installer and will fail to register WMHelper.dll, Ignore it. A Zune icon should appear on the desktop.
5. Apply the two Registry Patches “Zune.reg” and “ZuneShell.reg” as supplied HERE, by extracting then double clicking on them and selecting Yes.
6. Plug in your Zune. You should be prompted to install drivers. Follow the pictures below to install the drivers: (The pics shows to choose the option to manually select the driver location. Point to the location c:\Program Files\Zune\Drivers\Zune)
7. Now install the x86 package on a different PC (either Windows XP x86 or Windows Vista x86) and copy the contents of C:\Program Files\Zune (on the x86 PC) to the same location on the x64 PC. Delete the contents of C:\Program Files\Zune on the x64 PC before doing this. For people that don’t have access to an x86 PC I have compiled the files you need to put into the Zune folder (based on version 2.1.888.2) HERE.
8. Now register the Zune DLL files. Go to Start > Run > type “cmd” and hit OK. Type “cd\Program Files\Zune” hit Enter. Type “for %f in (*.dll) do regsvr32 %f” and hit Enter (enter the commands without quotes “”). You will get many popups. Some being errors some being successful. This is normal, click OK to all of them.
9. Run Zune.exe in C:\Program Files\Zune (or from the Desktop). Enjoy!
Credit to Grant “DigitalDJ” Pannell @ digitaldj.net for figuring this out.