A computer system for automatic replacement of advertisements includes an advertising server for selecting an advertisement based on criteria related to the individual viewer. In particular, advertisements are selected for a given user, based on the past behavior of that specific given user. Advertiser web sites on the network are configured to anonymously report back user activity such as visit dates, purchases, specific product pages visited and the like. Alternative reporting embodiments include email, file transfer protocol and spotlight tags. User activity lists are processed to select candidates for re-targeting. Candidates for re-targeted advertisements are identified based on their own individual past activity, and stored in a list of candidate user ID's. When a candidate on the re-targeted list is identified at any network affiliate web site, a re-targeted advertisement is delivered to the candidate user.