When installing a heatsink on an Athlon or Duron you MUST be sure that is is 100% flush. I generally do it this way:
First, apply a *tiny* amount of thermal compound to the cpu itself. Remove any "stock" compound on the heatsink itself.
Second, be VERY CAREFUL, slowly lower the heatsink and hook one side. (It's obvious which one.)
Third, very carefully center the heatsink and ensure you are flush with the die.
Now is the tricky part. Try by hand latching the clip to the other side of the socket. Often this will be impossible. If this happens, you will need to use a screwdriver or as someone else suggested (I like this idea, but haven't tried it) a wooden chopstick.
I know it sounds easy, but if the screwdriver slips you can punch a hole through the MB. If the heatsink isn't perfectly flush you can break a corner off the cpu die, AND not get good cooling.
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bloom25
-MAW-
(Formerly of the)
THUNDERBIRDS