Asus P4B - Manual (Page 30)

2-8 Chapter 2: Hardware information
2. Position the fan with the retention mechanism on top of the heatsink.
Align and snap the four hooks of the retention mechanism to the holes
on each corner of the module base.
Keep the retention locks lifted upward while fitting the retention
mechanism to the module base.
Make sure that the fan and retention mechanism assembly perfectly
fits the heatsink and module base, otherwise you cannot snap the
hooks into the holes.
Figure 2-10 Installing the Fan and Retention Mechanism
Retention Hole
Retention Hook Snapped
to the Retention Hole
Retention Lock