This is what is happening, if we assume that your A/V does remove the trojan, it is programmed to reappear. So even after removal it will return.
What can we do? If it were my computer I would try to use System Restore, which you have already said that it does not work.
Although not as insidious as a RootKit, a reappearing trojan can reappear at any time. Their true intent is to cause more harm than just giving your computer problems. If I were in you situation, I would reinstall. If you do not have a CD, but do have a key, I can get you an ISO (To burn a CD). Share your thoughts with me. When should I re-format? How should I reinstall? Security | DSLReports.com, ISP Information