The Athllon64 is completely compatible with WindowsXP. An added feature of the Athlon64 chips is that you can run a special (beta) version of WindowsXP (called WindowsXP 64bit) to take full advantage of the 64bit capabilities of the CPU. While operating in 64bit mode (i.e., runnimng WindowsX64bit), 64bit applications (a handful of games, video applications and math/engineering applications) will recieve a significant boost in performance meanwhile 32bit applications [in WindowsXP64bit] will recieve a minor performance drop because the CPU must operate in emulation mode rather than real mode.
That aside, the drop in performance often isnt the problem people have with WindowsXP64bit -- its the fact that many drivers and such are quite buggy in 64bit mode although I myself didnt run into any significant problems. If you are deciding between a AthlonXP and a Athlon64, I'd highly reccomend the Athlon64 (because the AthlonXP is aging/obsolete/discontinued technology)
