What I tend to do is place the computer behind a front seat and push it back until it's wedged between the front and back seats. At least in my jeep, this worked great for your average tower and kept them from moving at all. As for a monitor... Well if you have a CRT, I'd probably set it upside down (they tend to topple over) in the trunk or back seat. If you have an LCD, perhaps lay it down, screen up, in a floorboard, away from other objects that could fall on it.