The Associated Press reports that Hewlett-Packard Co. has extended its lead over Dell in the worldwide personal-computer market in the fourth quarter of 2006. Analyses conducted by IDC and Gartner, Inc. produced slightly different results, but the same general conclusion. While overall PC sales were uninspiring, HP managed to outgrow the rest of the market, and lead in PC sales for the second quarter in a row. PC sales increased by approximately 7.4 percent in the fourth quarter, and both firms noticed declines in the U.S. market, the second consecutive period with a domestic slump…