I have two computers, or four if you count my Xbox and a disassembled computer.
I have a Shuttle XPC, which is a small form factor computer. It is the black PC in the pictures.
As you can see it has two SATA hard drives at the top and an internal IDE HD as well as space for an IDE DVD ROM.
The Macintosh Mac Mini Core Duo is the white box with the Apple logo on top. Note the size when compared to the CD, pen, and mouse!
It is very small. It does this by using laptop components. ie 2.5″ HD, laptop DVD Burner and virtually zero space inside the case.
The box with the blue front to the right of the Mac min is an external IDE HD (@250GB)
The two boxes between the speaker and the Shuttle PC are an external DVD burner (USB/Firewire) and another external hardrive.