There are two major ways of detecting viruses in the computer.
These are:
(i) Using genetic checker: This checks sensitive areas of disk and files that are considered at risk. Principally programs files by storing information on such, it compares the newly obtained information about the disk and file and if there are any differences it will then give a signal of such, At this point, an antivirus can be used to fight against it.
(ii) Using Scanner: The program uses recognition string taken from the body of virus and looks for the pattern inside the boot sectors and files being checked.