My dell Dimension 4100 has a proprietary fan assembly that houses a 92mm fan and attaches to the case. It amplified the noise of the fan motor terribly, so I took the fan out and rather than drilling holes to attach the fan by itself, I used industrial velcro (blue peel strip) to do it and at the same time dampened the vibration noise of the case.
Space for the fan inside the case:
Hard side of velcro to the case:
Soft side of velcro to the fan corners and sides (make sure
the velcro doesn't interfere with the fan blades spinning):
The industrial velcro holds the fan extremely well and the adhesive has
the gripping power of duct tape:
WARNING: Do not use velcro to mount the fan on the heatsink