Some program viruses tack their virus code on to the end of the program. Others will look for empty spots inside a program. Both of these program file types can be cleaned up. Unfortunately, still other program viruses will replace program code with their own virus code. This means that these programs can't be cleaned up and must be deleted.
Some viruses are able to act as boot sector and program viruses.
The natas virus is a destructive boot sector and program virus. While older program viruses infect MS-DOS programs, newer program viruses now infect Windows programs. They may also talk directly to the operating system's e-mail handling instructions. Once they infect a system, the worm may look for an address book and mail itself to everyone listed in the book. Or, it may simply send a copy of itself to each recipient as you send out new e-mail.