Beth, may I suggest something since yours are the only computers this appears to be happening on. I know you said you have a scan running daily, but just so we can rule out an infection (if a virus attacks the scanner itself that can be problematic), could you go to http://www.trendmicro.com/housecall

That's an excellent "off-computer" scanner. If that doesn't find anything then at least we can be reasonably sure that's not the problem.