Which avast! products are compatible with Windows XP x64 Prof. edition?
All Avast products have been compatible with windows 64 bit edition for some time now
The only ones that you can use, Windows versions of avast 4 Home or Pro version are compatible with winXP 64, see http://www.avast.com/eng/x64.html.
What about servers ?
avast! Server Edition supports both x64 and IA64 (Intel Itanium) platforms