DENVER, Dec. 14 — The Denver police have gathered information on unsuspecting local activists since the 1950's, secretly storing what they learned on simple index cards in a huge cabinet at police headquarters.When the cabinet filled up recently, the police thought they had an easy solution. For $45,000, they bought a powerful computer program from a company called Orion Scientific Systems. Information on 3,400 people and groups was transferred to software that stores, searches and categorizes the data.