This is scare ware! It is trying to blackmail you for your money. You cannot do anything with your computer. Ctrl-Alt-Delete does not work. Safe mode does not help you. The only way that you can get rid of this is to pay the required amount which starts at £50. The computer will then be useable again…… for a few weeks and then it happens again.
I have now seen this three times and as part of a computer service it will be removed.