Well, I have first hand experience on this (no pun intended).

about 2 years ago, I was being sent as tech support for one of my then-employer's customers, to check out an outage of their internet access. The satlink diagnostics told me "blocked for excess upload", which meant that somebody had spyware/malware or was acting as a DoS-bot. And sure enough, by checking traffic there were a crapload of hits from one particular PC. After (phisically) finding the offending machine (it was in a separate area beneath the main office complex) I was going to run a basic virus scan when LO AND BEHOLD: the screensaver was on. And oooh what a smutty screensaver that was!

The second one was hilarious: some dude who had a win98 box at my previous job was complaining that his PC was way too slow. And I mean "You *** moron, why the f**k is my f**king PC not working as it should? Do your job right a***le!" kind of raging. Sure enough, 60% of his entire HD was full of porn, a collection that would make even some porn addicts envy this guy. So, being my last day at that job and all, I just left a note for everyone to see: "Your PC works better when you're not downloading PORN all the time!". I also said that to his co-workers (the dude wasn't there at the moment), loud and clear.

Remember, never mess with IT. We know everything you do. ;)