>Perhaps someone is using your computer to view pornographic sites, then deleting the history to erase the evidence?

Good, catch, people actually using someone else's machine to do their dirty work :-) A textbook case for tighter security on things like PW sharing...

Seriously, my IT people have tried to combat this sort of thing (porno, gambling, etc. from any machine) by writing site-visits to user mail/server drives. That didn't work well, though, the smarter users figured it out soon enough.

Now they simply site-block and monitor, much more effective. I don't like a lot of "big-brother," but I've seen some pretty serious abuse of web access, too. You have to do something about it in extreme cases.