Damn, what a bunch of sissies. Don't apologise for stuff that isn't your fault, tell them not to touch your screen. No need to make an excuse. Don't be rude, but don't be ridiculous over the top apologetic either.
Well, I think the reason most people try to be nice/humorous is that the screen-poker is generally not aware that they are doing something wrong. They view their computer as a piece of furniture--something that sits on their desks.
Making a joke or lighthearted comment is a lot more effective than saying "hey there, I know you just use your work computer for MS Office, solitaire, and Facebook, but I care more about this computer than most people care for their children. Keep your dirty mitts off of it."
They'll probably never understand why you care so much for your computer, so it's better just to pretend you have some 'weird' issue with people touching your stuff.
Here are more examples of the same technique:
"This is totally my own OCD, but it drives me crazy if..."
"Just a sec, I have to apologize, but..."
"It's probably just me, but it kills me when..."
(Don't forget to smile and be apologetic, tone is key)