These types of things are called "ransomware" and basically they try to lock you out of your computer and insist you pay a certain amount of money (say £100) for them to unlock it.
I have helped a few neighbours who have had this problem and in a few cases nothing I did would allow me to get past the "ransom" screen.
One thing that may help is to set yourself up a "spare" Windows user id so that if you get locked out your main userid you can log on to the "spare" one to run you security packages to see if they fix it.
In one case I had to download the Kaspersky rescue disk (for free) and boot from that and then run their security. This got rid of the problem.
This ransomware can be very nasty and can lock you totally out of your computer so be warned.
Read more here
http://www.microsoft.com/en-gb/security/resources/ransomware-whatis.aspx