Security Tool virus...need help removing

I downloaded Malwarebytes, and I had major trouble installing it. Every time the program would install, the Security Tool virus would delete the .exe file right away. What I ended up doing based on another person’s tip was to watch the install folder, and as soon as I saw the mbam.exe, copy and paste it to the desktop quickly. I did this, and then just moved the .exe file back to the program directory after the Security Tool virus had deleted it. I ran the program, and it seems like it took care of the problem. I ran Malwarebytes in both safe mode, and regular mode. No signs of the Security Tool Virus. However, after every scan of Malwarebytes it does keep showing 1 infected file that I tell it to delete, and upon restart and another scan, it still shows that 1 infected file…..I just figured maybe it was a glitch since the Security Tool virus appeared to be gone. After a day of my father being on the computer (it’s his machine I’m trying to fix) he said the Security Tool virus is back. All he has done is played Solitaire and check the weather on the internet, that’s it. I am running both Avast! Anti Virus, and Windows Defender and neither of these programs are finding anything. How is Security Tool keep re-installing itself?