Say you manually install Windows XP x64 into a VM, and configure a legacy network adapter. If you attempt to install the Hyper-V Integration Services Disk, you'll receive an error that the Operating System is not supported on Hyper-V. Also, in reading the documentation, you find that the legacy adapter (which normally appears as a DEC 21140 or an Intel 21140 PCI Fast Ethernet Adapter) is not supported on both Server 2003 x64 and Windows XP x64.
Just in case you NEED to do some testing with XP x64 in Hyper-V, follow these steps to grab the appropriate LAN driver for the Legacy network adapter for Windows XP x64 (might work on Server 2003 x64 also, but I haven't tested).
On a healthy Windows Vista Machine (x64 or x86, I tested on Vista SP1 x86), navigate to %windir%\system32\driverstore\FileRepository, and copy the directory named dc21x4vm.inf_7d8c6569 to a CD (be sure to include hidden files - 4 files total), or other media that you can access from the x64 VM. Then use Device Manager to update the NIC driver in the VM.
I also tested this in an Operating System Deployment scenario, using Auto Apply Drivers to install the NIC driver successfully.