An easy way to do this on a mac is to search for "newly added" software on the date that you got infected, the search will pull up all things added then, then it's just a task of deleting those files. An easy to do this if the virus is shutting down your window is to use the command prompt.
Otherwise I would use "Microsoft Security Essentials" it's free and I have used to to get rid of many trickier viruses, including the "anti virus soft" virus. What is the best feature of MSE is that most viruses will disable you from downloading anything, including hijack this or other anti virus programs. MSE ,however, downloaded and installed 100% with the virus trying to override it. The quick scan is the quickest way to clear the virus and the computer will run fully without issue but I suggest you run a full scan once the quick scan is done and you have more time.