San Francisco--ZDNet, Ziff-Davis' online technology portal, announced Tuesday it has chosen Lowe & Partners, San Francisco, to help it launch a new branding campaign, slated to debut by the end of this year. ZDNet said it plans to spend $25 million over the next year and a half. Competitor CNET is in the midst of a $100 million brand building campaign as it fights for visitors to its Web site. According to September data from New York-based Internet ratings service Media Metrix, the ZDNet sites ranked 15th, with 8.6 million unique visitors. CNET, meanwhile, ranked 17th, with 8.3 million unique visitors.