As of Zune 2, copying the Zune folder from Windows XP x86 to get Zune to work in Windows XP x64, no longer works. It now requires a little bit of extra tweaking.
The x86 package still needs to be used. The x64 version is Vista only and relies heavily upon Microsoft Media Foundation (for Vista only).
Support for this hack can be found at the
ZuneBoards forum post
Installation How-To:
Note: Drive C references your Operating System drive.
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.

3. Download the
Zune Vista x64 Package and extract it using a decent UnZIP application. I recommend
WinRAR.

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:

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 2.5.447.0)
HERE.
Your Zune folder on the x64 PC should look like this:

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!
Note: Zune may complain about not being updated even though it is. If you want to update you will need to check for updates on an x86 PC and then repeat the process.
Brought to you by Grant "DigitalDJ" Pannell @ digitaldj.net