I cleaned out this system because it was so old and thick with dirt but it's the most unusual pc I have ever seen, I can't figure out how to put it back together, the part that is giving me the most trouble is the heat sink, I have pictures here:
The bottom of the motherboard, the heatsink from several angles. Obviously the fan clips to the heatsink and it may be the other half of the mounting mechanism? Does the fan look like it has any posts that might go in the four large holes on the motherboard?
If you provide the model of the system you could look up ther service manual. Stock images of it, or look for replacement parts that might have diagrams of how it is to be installed.