It will work just fine for a A XP system. The reason it says P4 is it has an additional power adaptor thingee that the P4 boards need. All the Athlon XP boards need is the normal mobo cable (though some also make use of the small 4-pin power cable as well).
-spike
EDIT** you might want to check out other PSU's as well. While that one looks good, it only has 5 molex connectors and may not be the best brand google the name to fine some reviews. If you are looking for a decent PSU for not much $$ then you could also look at the Thermaltakes, cheaper Enermax's, or even the Enlights. The TT and Enlights are not the best of the best, but they are definitly good enough for most systems.