Ubuntu :: Make Computer Accessible Without Opening Gaping Security Hole?

my son is 15, autistic and mentally retarded. he is moderate-low functioning and loves to watch barney the dinosaur, thomas the tank engine and other young children's programming on videos on his computer. he can talk in a limited way and can read out loud at about the first-grade level, though he seems to have little or no comprehension of what he's read (hyperlexia).i'm having problems with viruses and spyware because he lacks the judgment that keeps the rest of us from clicking on every window that pops up. we've had a particularly nasty crop of ad-ware viruses lately that pops up graphic porn ads even when the browser is closed. i've had enough of this.

the computer is an ancient dell dimension 4600 desktop (circa ~2002) running windows xp. i've run ubuntu from a live cd and installed flash as a test. videos videos play fine, so that's not a problem. i don't really want to replace the computer becasue it still works and is only used by him to run firefox. i am willing to buy a new computer if that turns out to be the only option.i've been using ubuntu exclusively on my laptop for several years and i would like to remove windows from his computer and replace it with ubuntu. i'm wondering how i can make his computer as accessible as possible to him while not sacrificing too much security.my son has very poor fine motor control over his hands. he can use a mouse with some difficulty, but using a keyboard is out of the question. he can use the mouse to click on the shortcuts to his favorite videos, but i can't think of how he would be able to enter a password for his account. not only does he have the fine motor problem, but he is not able to remember any usefully secure password.

does anyone have any ideas about how to make his computer accessible to him without opening a gaping security hole? two ideas that i've kicked around are creating a user with absolutely the minimum privileges required to use firefox and no password or finding some way to enter a password that doesn't require a keyboard. i haven't come up with how to implement either of those ideas successfully.