A method of, and system for, virus detection has a database of known patterns of start-up code for executable images created using a collection of known compilers and uses examination of the start-up code of the image by reference to this database to determine whether or not the executable image is likely to have been subject to infection by viral code. In particular, the system seeks to determine whether the expected flow and execution of the image during start up has had viral code interjected into it. Various heuristics to assist in assessing the likely presence of viral code are disclosed.