A system at a web site addressable by one or more client computers. Each client computer connects to the web site to receive a respondent identifier and a viewer. The viewer at the client computer generates a unique viewer identifier identifying the client computer, and sends that identifier to the web site for registration with the respondent identifier. The viewer connects to the web site and downloads a file with encrypted content information. The viewer requests the web site for a key to decrypt the file. Based on the respondent and viewer identifiers, and the particular survey and exposure limits, a key is sent to the client computer. The viewer decrypts the encrypted file with the key, and then shows the content information in a window. During viewing, the viewer ignores interrupts from the computer's user interface enabling copying, and stops showing the decrypted content if another window is selected.