You want a 3 pin fan and just cut one of the lugs off to make it fit, some one will tell you which one to cut off, the 4 pin header on the HP motherboard isn't a standard pwm so you have to also modify a 4 pin if you use one of those.
Have a look at this post by maddness http://forums.overclockers.co.uk/showpost.php?p=24671250&postcount=6457
Thanks, I've order the "kit".