how to get rid of trojan virus using system restore

I read that using system restore with disk defragment tool would eliminate Trojan virus, but after reading the trouble some are having not sure which restore point to use etc. worried I my knock out my system instead of cleaning it. Advise please, Oh, I downloaded AVG 7.5 and its working but it backed up the virus and I had to manually delete, it returned to show up only in virus history. Not sure where I’m at with this.