A virus detection system (VDS) (400) uses a histogram to detect the presence of a computer virus in a computer file. The VDS (400) has a P-code data (410) for holding P-code, a virus definition file (VDF) (412) for holding signature of known viruses, and an engine (414) for controlling the VDS. The engine (414) contains a P-code interpreter (418) for interpreting the P-code, a scanning module (424) for scanning regions of the file (100) for the virus signatures in the VDF (412), and an emulating module (426) for emulating instructions in the file. The emulating module (426) contains a histogram generation module (HGM) (436) for generating a histogram of characteristics of instructions emulated by the emulating module (426) and a histogram definition module (HDF) (438) for specifying the characteristics to be included in the generated histogram. The emulating module (426) uses the generated histogram (500) to determine how many of the instructions of the computer file (100) to emulate. The emulating module (426) emulates (712) instructions and the HGM (436) generates a histogram of the instructions until active instructions are note detected. When active instructions are not detected (714), a P-code module is executed (722) to analyze the histogram (500) and determine whether a the file (100) contains a virus. The P-code can also decide to extend (728) emulation. The HGM (436) is also used to detect (822) the presence of dummy loops during virus decryption.