Got the same a few weeks ago, was looking at replica breitlings as i don't want to wear my real one to work, was on a site from the netherlands and up it popped 'your computer has been locked', metropolitan police logos all over it and also instructions to follow on how to unlock it, just nip and buy a voucher and enter the code into a box on the screen to unlock.
At first i crapped myself but then thought hang on, if the met has deemed it serious enough to lock my pc for looking at dodgy sites how come i can just pay the money,buy a voucher and they'll just unlock it as if nothing happened, i googled it and found pages of others that got the same
And it originates from the netherlands
Just to add it got past McAfee in the first instance but did a full scan and it cleared it, maybe just turning off and on again would of cleared it too