In the past, It was never a good idea to run two antivirus scanners simultaneously. However, if only one of the scanners has real-time protection enabled and the second scanner is used only to manually scan selected files, they may possible co-exist peacefully. Some Software like Avast Antivirus Pro Crack is designed to be the 2nd scanner on a system. In a few cases, an antivirus scanner will not install if it detects another antivirus scanner already installed on the system.