Does anyone know how to get rid of a virus on the computer?

I have Norton Virus Protection but sometimes it doesn't remove the problem. The virus is called MBS microbill I tried everything but it keeps comming up and blocking everything for 10 minutes or longer.It happened the first time saying that I registered for a website and need to pay �20 which I did but now it says I owe them �99.65. I cancelled the membership first time why is it comming back? Please help me as I don't know what to do.Thanks