Insane Virus - Advice

OK. So I traded my brother for his desktop not about a week ago. I was told there was a virus on there and all I needed to do was get an anti-virus and I would be good to go. Got an antivirus, ran it and it "Cleaned the viruses". Computer ran fine for a few hours. Logged back on next day and the computer was deathly slow... took about 10 minutes to open control panel. Opened anti-virus, the antivirus was turned off and any attempts to run it resulted in an error. Quarantine was in Spanish, showed 8 files in quarantine (in Spanish), but when report was opened it showed nothing. Got a cloud operated antivirus which detected and removed the viruses. Worked well again. Got on the next day, same thing. Got AVG antivirus, does the same thing to that antivirus. It seems to be replicating itself from somewhere even after it is removed. Affects restore, antiviruses, and general computer use. There was no CD with it to restore it to factory. Not sure what to do and definitely open to suggestions.