Xerox Corp. said it is spinning off a company that will market flexible, paper-thin screens for in-store advertising that can be updated using a wireless computer system. Xerox estimated Wednesday that Gyricon Media Inc. will generate revenue of more than $100 million annually by 2004. In-store computer-controlled signs identifying products and prices are expected to reach the market late next year. Billboards, electronic books and even computerized newspapers are future products envisioned for the Xerox technology. Shares of the Stamford, Conn.-based copier company closed off 13 cents at $4.63 on the New York Stock Exchange.