In my old company I said that people can use their computers for personal stuff too, as long as it was legal and wouldn't compromise the company in any way. I was fine with that. If people are spending 10-16 hours per day in the office, get your bills done, send email to your spouse, watch your nanny cam, look up what those little bumps might be caused by, or whatever. Just get your work done too.

I always made it a policy that whenever I stepped behind someones desk that I asked first. You'd be surprised how far respect goes amongst professionals.