"Pay to unlock your computer" scam

Has anybody else suffered this scam?
Your screen freezes and a big message appears telling you your computer has been locked because you're guilty of cybercrime, and that you must pay a fine of £100 to get it unlocked. Needless to say we're innocent.
Ctrl-Alt-Del won't get us out of the locked screen, so we have to switch off the computer, but on rebooting, the lock and message is still there!
I rebooted again, this time in Safe Mode and ran a Sys Restore which got rid of the message and unlocked my computer, and I followed up with AVG and Adaware scans, and my PC seems to be working fine again.
I only mention it to let people know they'll be able to do what I did and use Sys Restore to get out of the scammy lock.
I'm not too PC-savvy so there may be other ways to break the lock that i don't know about besides Sys Restore, I don't know.

Sounds like some variant of Ransomware, a form of extortion in which the attacker encrypts a victim's personal information (data files) and then demands money (ransom) in exchange for a decryption key that can be used to retrieve the encrypted files. For more information, please read:

• Ransomware ‘Holds Up’ Victims by Sravan Ganachari
• New GpCode Ransomeware attack by Harry Waldron

Sometimes using System Restore as a method of recovery works but other times it may not since System Restore was not designed to be a virus or malware removal tool. Whether it will be successful depends on what type of infection you are dealing with, what damage the malware has already caused, whether it disabled System Restore and if not, what is restored during the process.

This is what mvps.org has to say:
Can I use System Restore to remove virus or malware infection?

NO. System Restore was not designed to be a virus or spyware removal tool and should not be depended on.

Kaspersky Virus-fighting utilities have several fixes for some types of ransomeware such as:

• XoristDecryptor (Trojan-Ransom.Win32.Xorist)
• RectorDecryptor (Trojan-Ransom.Win32.Rector)
• RannohDecryptor (Trojan-Ransom.Win32.Rannoh)

Okay thanks, the Ransomware subject is more complicated than i thought, so I was lucky that Sys Restore plus AVG and Adaware scans seemed to get me out of trouble.