You are probably going to have to manually research the files Prevx flags, and delete them, and the associated reg entries. (Carefully. You can do damage with the registry.) You're in for quite a learning experience. I can't help with this from afar, sorry.If it was my computer, and I could see what was happening, I'd have a go, but it's yours, I can't, and it would be irresponsible to try.
Someone else here with a bit more expertise may be able to help, especially if you post the Prevx scan report.