Remove JS:Redirector-BOB [Trj] - Manual Removal Instruction

JS:Redirector-BOB [Trj]  - Infection Detected - Avast! Web shield has detected a treat.
Since this afternoon the following infection has been detected on my imac and every 30 mins or so I have a popup informing me of the following "JS:Redirector-BOB [Trj]  - Infection Detected - Avast! Web shield has detected a treat." 
Does any one know if my computer is at risk? 
What is the JS:Redirector-BOB [Trj]? 
How do I remove the Trojan from my mac.

JS:Redirector-BOB [Trj] has targeted Mac users by redirecting them from legitimate websites to fake websites which tell them that their computer is infected with a virus. The user is then offered fake message to solve the issue. However, if you follow the fake instruction, your mac will be messed. Its ultimate goal is to get the user's credit card information which may be used for fraudulent purposes. Apple released a free software update that will automatically find and remove the real virus and its known variants. The Resolution section below also provides step-by-step instructions on how to avoid or manually remove this infection.