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.
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.