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):