PC shipments world-wide dropped 0.4% in the fourth quarter from a year earlier, the first year-to-year drop in six years, according to IDC. Rival market-research firm Gartner Inc. offered a slightly more upbeat estimate Wednesday, reporting that fourth-quarter shipments world-wide rose 1.1%. PC sales in the US were particularly weak, with shipments falling 3.5%, according to IDC. Gartner said US shipments dropped 10.1%.