Computer viruses are the small programs which created by some advance level programmer to harm the computer.
There are lots of categories of viruses like Trojans, Adwares, Spywares etc etc. but the work of all of them is same to harm users by damaging the computer or by leaking out some sensitive information.
If you got any virus in your computer so the first thing which you have to do is just SCAN your computer with your ANTI VIRUS.
In case if your Anti Virus program is not working fine or not able to remove virus from your computer so you can use these free tools or trial version tools to scan your computer.
Malwarebytes is a light-weight anti-malware program that is excellent at removing the latest detections
Download Link :- http://www.bleepingcomputer.com/download/malwarebytes-anti-malware/
SUPERAntiSpyware is a free anti-spyware program that offers excellent detections and quick removal of common infections.
Download Link: http://www.bleepingcomputer.com/download/superantispyware/
Trojan Remover. Not only detects Malware files but automatically removes them, without the need to use DOS or SAFE mode
Download Link: http://www.simplysup.com/tremover/download.html
(formerly Hitman Pro) is a computer program to detect and remove rootkits,trojans, viruses, worms, spyware, adware and other malware (anything that may harm a computer), using extensive behavioural analysis and cloud computing. The program is developed by SurfRight.
Download Link: http://www.surfright.nl/en/downloads/
However in these days I got complaints from the customers saying that they are infected with a Virus which completely locked down their computer screen and giving them a warning from Federal Bureau of Investigation or etc etc. and asking them for money to unlocked the computer.
If you are infected with this Kind of infection so here is a complete step to step guide to remove it.
REMOVE FBI INFECTION