This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.
Indication of Infection
Michelangelo is triggered on March 6. The payload on this date is a reformat of the system hard disk by overwriting any data with random characters. This occurs in the first 17 sectors of the first 4 sides of the first 250 cylinders, or approximately 8 MB.
Total system and available free memory, as measured by the DOS CHKDSK program, typically decreases by 2,048 bytes.
Methods of Infection
The only way to infect a computer with an MBR/Boot Sector infector is to attempt to boot from an infected floppy diskette. The boot sector of the diskette has the code to determine if the diskette is bootable, and to display the "Non-system disk or disk error" message. It is this code that harbors the infection. By the time the non-system disk error message comes up, the infection has occurred.
Once the virus is executed, it will infect the hard drive's MBR and may become memory resident. With every subsequent boot, the virus will be loaded into memory and will attempt to infect floppy diskettes accessed by the machine.