New
#11
So on review (its been years since I saw LGA775), the original HSF just plugs into holes in the motherboard.
The posts you have may are for a custom heatsink (yours looks different from the standard Intel HSF) by your PC manufacturer, implying the presence of a special backplate.
You will have to remove it, which means getting the board out of the case, I am afraid.
You just place the TX3 mounting clips in the holes in the motherboard and push down to lock the clips in place.
If you are uncomfortable with rwmoving the notherboard you may have to approach your local PC repair guy.