Privacy at Work: Where to draw the line?

So here's a pet peeve which might not be a very personalized pet peeve. Have you ever arrive at work one day and realized someone has been using your computer? Sending out emails using your emails? Talking using your Skype account (when you stepped away for to use the restroom)?

WELL. IT IRRITATES ME TO THE CORE.

And it happened more than once AND ITS STILL HAPPENING.

If you've a similar pet peeve, here's a wallpaper that you can use. Hissing when the offender steps near your workplace only helps to a very small degree. May try spitting and raising my claws the next time. Do let me know what measures you will take.