i have xp64 running on my render nodes and it's working great!
It is NOT slower than win7 and it consumes LESS memory than 7. Which is good for rendering large scenes.
I have Kaspersky IS running on them for years without any problems - no workarounds!
The only thing that can be problematic with xp64 is the printer driver support and pdf writer compatibility because it installs some kind of printer too.
But if you need it for a workstation i would take win 7 x64 because it "feels" much better. I have 7 x64 running on my workstation (24GB) and macbook pro (4GB)