Yeah I had that same problem. From what i can tell, and I'm no expert here mind you, it seems that when Apple released their new version of iTunes at the end of June they decided to drop all x64 support. t also coincided with a Windows update. Between the two iTunes stopped working for all XP x64 users.
I had still had the previous version of iTunes on my computer and so I uninstalled iTunes, reverted my system back to before the Windows and iTunes updates (6/28 or so) and reinstalled iTunes from the previous installer. It's working fine now. I'm afraid to update Windows and refuse to update iTunes, so obviously this isn't a permanent fix. However, from what I could tell, there was quite the uproar on the Apple site due to the dropped support, so I expect that future version of iTunes will work on XP x64.
Good luck and may the Force be with you.