You should alert your users that a particularly effective scam is growing by leaps and bounds recently. It's not new, but it's bursting into mainline cybercrime these last few weeks. The scam takes over the full screen of the PC, stating that the FBI has locked that PC until a fine is paid. The PC may look locked down, but it was a cyber criminal who did that, not the Feds.
What to do: Do NOT PAY, this is malware on the PC. Treat it like malware and clean that system. The bad guys have found this is a scam that works really well for them. Scared PC users are often willing to pay hundreds of dollars to avoid getting in hot water with the FBI. More than $5 million per year is extorted from victims. If it's a PC in the office, call IT. If it's a PC at the house, here is a video from security company Symantec how to remove this for free: http://www.youtube.com/watch?v=_dKBXeoLIFo.