This file can contain device drivers, executable files and system parameters. Your computer is up and running, but your virus problems aren't over yet. After the initial transfer to the boot record, viruses have a number of opportunities to infect the system. It's interesting to note that some viruses can tell they've already infected a program while others cannot. It's the stupid ones that, after a while, are easy to spot.
Polymorphic viruses are anything but stupid.
Polymorphism means the occurrence of different forms, stages or types. And that's what a polymorphic virus does. It changes itself to avoid detection. Programs possess certain binary characteristics called signatures. These can be anything from the file header to a particular series of instructions.