Virus.DOS.Enter.613 or Enter is a virus that runs on MS-DOS.
Enter is a relatively harmless memory resident parasitic virus. It hooks INT 9, 21h and writes itself to the end of EXE files that are accessed. By hooking INT 9 (keyboard) the virus calculates ENTER keystrokes and on the 100th ENTER reboots the computer. While not damaging, it can get annoying after a while and can even make one lose work unexpectedly.