A type of application or device driver that is initially loaded into RAM and remains active while the system is running. Also known as a memory resident program. A virus attaches itself to a file or boot sector and replicates itself to other files, disks, etc. A third type, Multipartite, are viruses that exhibit behaviors of both Boot and Program viruses. Also with the recent development of Macro Viruses, document files such as .doc or .dot are also susceptible to program viruses.
Boot viruses attack the boot sector of a disk.
The boot sector is where the information required by the computer's hard drive and operating system to start up the computer is permanently stored. Boot viruses normally operate by supplanting the existing Master Boot Record located in the Boot Sector with its own viral code. This code is then read into memory every time the computer boots up, allowing the virus to deliver its payload while continuing to infect other disks that come into contact with the computer. Program viruses attach themselves to an application or document file. While in memory the program virus can deliver its payload and infect other executable or document files.