• Install the x64 Windows Media Player 11 (looks like Zune is basically a skinned version of WMP11). You need a genuine version of Win 64 to install WMP 11 (install it from Microsoft here: http://tinyurl.com/tcvck ).
• Install the zune software on a 32bit machine, go to c:\Program Files\Zune and get the Zune.inf driver file. Zune doesn't seem to actually have a binary driver, but instead customizes the one that comes with WMP11.
• Copy the zune.inf file and make the following changes:

Replace:

[Manufacturer]

%MfgName%=Microsoft,NTx86

[Microsoft.NTx86]

%Zune.DeviceDesc%=ZuneMTP, USB\MS_COMP_MTPZ

With this:

[Manufacturer]

%MfgName%=Microsoft,NTAMD64

[Microsoft.NTAMD64]

%Zune.DeviceDesc%=ZuneMTP, USB\MS_COMP_MTPZ

• Then you plug in your Zune, point the driver to the zune.inf file (you don't need any other files). It will complain about the driver not being signed because you messed with the inf file. Or you can just right click the file and select "install".
• xcopy the c:\Program Files\Zune directory from your x86 machine to the your c:\Program files (x86)\Zune dir on your 64 bit OS.
• regsrv all the DLLs in that directory. Press win+r, write cmd, on the command prompt that appears write:

cd "\Program Files (x86)\Zune"  (withe the " character at both ends) [press enter]

for %f in (*.dll) do regsvr32 %f    [press enter]

About 25 dialog boxes will come up, press OK to all of them (some will be errors, but that's ok)

• Open windows explorer (win+e), go to c:\Program Files (x86)\Zune . Right click on Zune.exe and Send To -> Desktop (create shortcut). 
• Run Zune! It will complain a bit the first time, but you should be able to sync and rip now.

Zune Software on Windows XP 64 Professional Screenshot (click on the image for 1024x768 version):

Thank you Kristof for the hack.

EDIT1: After some days looking for him we found kristof's space. 

EDIT2: The registry keys you had to replace had a tiny typo. It has been corected. Thanks APOLAUF for the tip. 

EDIT3: This trick works also with the v1.2 Zune Software. Our forum member prtsmgr posted a screenshot .