now select the polygons that you want to be the holes and delete them. After that select all the borders of the deleted polygons and shift + drag them back. This should be the result:

Do the same one more time to create more holes for the ventilation of the case.

Now work on the places for the video card, sound card etc.

It is time for the fan start by creating a plane and giving it a shape like that in the picture. After that extrude it and chamfer some of the border edges.

Then add a cylinder and copy the fin several times. Everything else do just like in the picture.