I don't use sitters very often, but I've been surprised that each one, unless specifically told that the computer is off-limits, has helped herself. Although nobody visited inappropriate sites (I checked.), and I'm fairly certain that the use took place after the kids' bedtimes (because the computer is in an isolated space and my 2 1/2 year old tells me everything that he does with the sitter, so it appears that the kids are being well attended-to), I find it surprising that these girls have felt that they could use my personal computer without asking permission.
I've recently set up a password requirement in order to use the computer, although I haven't had anyone babysit since I have done so. I just feel like my computer is a personal item. They can use my television, DVD player, electronic piano, or bring a book for after bedtime. Do you think that I am being unreasonable to expect that someone not assume that it's fine to use my computer, even after the kids are in bed?