There's a particularly nasty little virus going around called "FBI MoneyPak Ransom". We got it with it, and it's been a headache to get rid of, so if you would like: Copy down the instructions on this link in case it happens to you. You might also want to go ahead and download the recommended cleaners there so you don't have to mess with it so much at the time.
The virus looks like this:
It is meant to look "official".
The upper right hand corner has your IP address, internet provider and location. I blocked ours out to put it here. There's also a pornographic photo where the yellow box is (I also blocked that out).
You will probably have a hard time exiting the forced lock screen, so follow these directions carefully and install the recommended stuff now if you have the chance: