Virus Downloaded Trojan

Hi all...

 

I got a MacBook Pro with Lion 10.7.3 and my Norton Antivirus detected a Virus "Downloaded Trojan" in two places, I tried several times to repair or delete... (The infected file that the AV reports is located in /Libreria/Application Support/Symantec/Antivirus/QTFiles and the other s located in MobileBackups...

 

I did full scans after Norton corrected the problem but I keep receiving the report that files are infected and I get asked if I want to repair, repair all or delete

 

Please does any one knows how to finally erase and have the computer cleaned up?

 

Thanks in advance