A system is provided to monitor a user's interaction with a computer. The system may comprise a reference string generator to generate a random reference string, an image generator to generate an image including the random reference string, a communications module to communicate the image to a client computer for display to a user and to receive user input data and a comparator to compare the random reference string and the user input data to detect human interaction with the computer. The image including the random reference string may be generated such that each character in the random reference string is off-centered.