I've had a long-standing policy that after using someone else's computer, I wash my hands with soap and water. I assume that this is less offensive than, say, snapping on a pair of surgical gloves prior to sitting down at my co-worker's desk.
The thing is, I realize that people sit at their desk all day long, eating, sneezing, coughing, and doing who knows what else. The direct recipient of all these activities is: the keyboard! And his ugly brother, the mouse.
Well, this has only confirmed my suspicions. Maybe those surgical gloves aren't such a bad idea after all.