Two robbers get into your house. They take all the light-weight items and just before they go, they try to take some more heavy stuff with them [your pc for example]. They try to take it out, but the pc is damn tighten with nuts and bolts. As usually, robbers try to be fast, so when they see the pc isn't moving, they will just say "screw it" and left out the apartment/house. No time to find a screwdriver and other tools to unscrew a pc.
Step 1: You Will Need
1) a drill
2) 2 bolts and 2 nuts
4) desk [wooden, steel or aluminum]; glass is hard to drill, i guess.
Step 2: Action
1) Do your measurements and markings as accurately as possible - adjust your computer on a convenient for you place, so you can open the side panels easily, there is a free space behind the computer [so the psu can "breath" freely] and you can easily and fast remove the screws when need to take out the computer.
2) drill two holes on the desk
3) drill two holes on the computer chassis bottom
4) attach all the usb and other cables, while the computer is still moving freely
4) then place the bolts
5) put nuts on the bottom side and tighten on hand only [it is enough].