If your FBI virus, as you call it, is anything similar to the Scamware or Ransomware attacks we get in the UK, purporting to come from the police authorities and "fining" you for going to certain websites, it needs a lot more than a simple anti-virus to get rid of it. If that were to work, the scam wouldn't have got past your AV in the first place.
These things just attach themselves to websites and when you first see, it just comes as plain text saying "come to our wonderful website and pay us a £100/$100 50000Yen" or whatever. No AV can see that because it doesn't contain any malicious software code.
What does your FBI attack actually do to your system?