Let's say you have this hypothetical situation: A person runs an program (that would put a virus on a XP/Vista PC) on a Windows 7 (important) PC. Would the virus still be attached to the computer if it didn't 'fully' run? For instance due to the compatibility issues between XP/Vista and Windows 7, the program turns up an error and a missing file dialog. Would the PC still be infected with the virus? The reason I ask is because my friend ran into this situation and he wants to know if his PC is infected with the virus. He ran Windows Defender and everything seemed okay. EDIT: MY friend just told me an additional bit of info. He ran the program from his USB drive.