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Sherlock's Computer Nearly Taken Hostage
This is yet another good reason to have a second account on your computer. Don't use it for anything except to use it when your computer is badly infected, installing the malware checkers after setup if they're not there already. The advantage is that when the original account is infected, the second account should not be affected.
We're familiar with Windows Security Suite and its related ilk and copycats. Now meet Reveton and its creator software.
Reveton Malware Freezes PCs, Demands Payment
Not exactly new, but Reveton builds upon previous crimeware and rasomware by adding in a twist of acting like the FBI's taken over your computer. The FBI will never do this. They will serve you a warrant/subpoena, arrest you, and/or take the computer.
In this case of infection, malware checkers should be able to clean this infection, but in some cases, a complete format and reinstall is better.
Here's an updated list of what to use. All are free:
Microsoft Security Essentials - http://windows.microsoft.com/en-US/windows/products/security-essentials
CCleaner - http://www.piriform.com/
Glary Utilities - http://www.glarysoft.com/
Malwarebytes Antimalware - http://www.malwarebytes.com/
Superantispyware - http://www.superantispyware.com/
Spybot - http://www.safer-networking.org/
AVG - http://free.grisoft.com/
Microsoft Security Essentials can easily work on its own, though you may want to use the others to help out.