The other day my mom asked me to have a look at her laptop. Apparently her copy of windows had decided that it was "not genuine."
After poking around and verifying that she had the same key I installed it with originally (which I know to be genuine) I finally discovered the cause of the problem:
Ah, very good, Microsoft! Tell people that they're stealing from you rather than telling them that their clock is wrong. Some people are extremely sensitive about being told that they have done something illegal. And it was in no way obvious how to find out why you've just been accused of software piracy. I'm pretty good with computers (even your crappy OS) and it took me a good 15 minutes to figure this out. Most of your user base would never figure it out.
This is the kind of thing Microsoft needs to get sued for.
Update: The clock was off by more than a month. Fixing the clock and rebooting twice fixed it. My point, though, is that you should tell the user what's wrong, not jump to conclusions.