While some site's security certificates really will lapse, and you'll get that kind of error, if it's happening to large number of sites, strangely enough a fix for that is sometimes to make sure that the time and also date are set properly on your computer. (Be sure to double check the date) There's other things that cause it, and hotfixes and things like that to try, but the time/date is by and large the culprit. Otherwise, everything from Internet Explorer settings to operating system bugs and other things will cause that error as well.