A computer virus is a very small program that hides inside another program. When an infected program is executed, the virus code is executed, together with the original program. A virus usually infects other programs or disks and then performs some other kind of task.
Viruses can sometimes be very damaging.
Scan your computer and floppy disks for viruses. It's always safest to cold boot from a clean, write-protected system disk and then scan. OK, I have a virus. If you didn't cold boot from a write-protected floppy, do so now and scan the system again. If the virus is in a file and you have a clean copy of the file somewhere else, then delete the virus-infected file.