On my laptop I have been running XP 64 bit edition for some time, by and large this presents no problem and allows me to successfully use all of the 4G of memory installed. However every now and then I come up against the odd road block because it is something of a second class citizen in terms of the support it receives from Microsoft and other vendors.
A good example of this is that when trying to install the latest release of Windows Live Messenger (because it supports multiple sign-ins, at last) I discovered that the installer and web site say it is only supported on XP 32bit. However it turns out that the only bit that is really not supported is the Windows Live Essentials installer that tries to ram all the other Windows Live / MSN features down your throat.
In short you can download the various cab files required for each component and install them by hand, instructions and links to the components are here. Hurrah!