Virus.DOS.Alcom.257 or Alcom is a virus that runs on MS-DOS.
Items that have this flag are generally malware such as viruses, trojans, hijackers, spyware but could also be programs that are not desirable to run on your computer.
This is known to be dangerous in almost all cases.
Removing Alcom.257 manually is hard. You never know how many files it has infected.