The boot sector on the hard disk controls how your operating system starts when you turn on the computer. A boot infector replaces the disk's original boot sector with its own. Once the viral boot sector is read, it is loaded into memory where it may spread and inhibit system performance. Boot tables are stored on a diskette or hard drive and tell the computer how files are stored on the disk. A virus will change these files or move them somewhere else on the disk.
Multipartite viruses have abilities of both program and boot infector types.
Once a computer is infected, a virus may display several different characteristics. The virus loads into memory with the host program and stays resident when other programs are executed. In memory, it can easily replicate itself into boot sectors or subsequently executed programs. This is the most common virus characteristic. The virus does not stay resident in memory after a host program is closed.