Hewlett-Packard has extended its lead over Dell in personal computer sales according to two industry reports released Wednesday.

HP's worldwide PC market share grew to 17.6 percent in the first quarter of 2007 with sales of just over 11 million units, according a preliminary report from Gartner. That's a 28.7 percent year-over-year increase in PC unit sales. In the first quarter of 2006, HP's share was 14.9 percent on sales of 8.6 million units.

Dell, meanwhile, saw its share drop to 13.9 percent, from 16.4 percent in the year ago quarter, based on a 7.8 percent decline in unit sales to 8.7 million.

A report from IDC also shows HP gaining share at Dell's expense, though with slightly different numbers. IDC says HP's share grew to 19.1 percent from 16.5 percent in the year ago quarter and Dell's declined to 15.2 percent from 18.2 percent.

The IDC and Gartner reports do not include sales based on revenue.

Overall, PC sales rose 8.9 percent worldwide to 67 million units in the Gartner report and by 10.9 percent to 58.9 million units in the IDC report. IDC also notes that first-quarter growth exceeded its forecast of an 8.5 percent increase. IDC said the growth was stronger in the portable PC and consumer markets than in commercial markets.

But the two companies, which define the term 'PC' in slightly different ways, differ on the market share rankings of the rest of the top tier vendors.

Gartner ranks them as Acer (a 6.8 percent share), Lenovo (6.3 percent share) and Toshiba (4.1 percent share). The way IDC sees it, Lenovo and Acer tied for third (6.7 percent each), and Toshiba followed with 4.3 percent. IDC reports that Acer and Lenovo each sold 3.969 million units in the first quarter.

Although past releases of new Microsoft operating systems could be counted on to stimulate PC sales, both reports stated that the release early this year of Vista caused little overall impact on PC sales.