There are some things you might wish to consider based on physical security.

A CD ROM/Floppy/USB port offers a route to get malware onto the system.

A case that can be opened can have other hardware added (eg ADSL modem to allow ecternal access) or the hard drive to be removed and cloned, allowing the system to analysed/craked at leisure

A computer that can be physically carried out of the room allows the system to be opened and hence the above applies

Etc, etc