P6 is fully compatible with all the P4 and P5 content. You may have geometry issues (backfacing polygons and ballooning prop geometry if you try to use smoothing on it), but they can be worked around. I believe there =is= a workaround for this; it has something to do with the security checkbit that the Athlon 64 implemented, that was enabled with SP-2. But not having XP (still with trusty ol Win2k), I can't say what it was. One of the XP techies about will have to take it from here...