There are two types of antivirus software, Windows and DOS version.
I was told that some virus locate themselves in the memory during the Windows startup, then most Windows-version antivirus softwares can't kill these virus, the only way to delete them is to boot the computer from floppy drive(or other removeable drive) where there is a DOS-version antivirus software, that is, only in the DOS environment, the virus can be cleaned up, not in the Windows.
I am not sure if this kind of remarks is correct, Thanks for any help!