I guess I should have been more specific. There was no need to remove the heat sink to clean the fan, you could have done that with a brush or compressed air can. You only remove the heat sink to change the thermal paste. If you removed the heat sink you MUST remove the old paste using 99% isopropanol or ethanol and apply new paste. If the plastic fasteners are broken, you should probably get a new CPU fan. They are rather difficult to use, unfortunately, and I had a hard time removing and replacing it. I did write some notes on it tho:
Turn in = turn tabs so that arrows point inward
Turn out = turn tabs so that arrows point outward
Turn in one by one
Pop out the heat sink
Turn all tabs in
Pull all tabs out / up so that pins on bottom are inside the white plastic (unexpanded)
Turn all tabs out
Place heat sink and push in the tabs slowly, one by one, in opposite pairs
LEAVE TABS TURNED OUT !
Even so, it's difficult to get it right. Maybe other fans have more sane attachment methods.