New numbers from the NPD research firm show that unit sales of Windows 7 packaged software boomed in the U.S. immediately following its Oct. 22 launch, far outpacing sales of Windows Vista in the first days of its January 2007 release. But Windows 7 hasn't done as well as Vista initially did in spurring new PC sales.

Complicating matters is the fact that Windows 7 and Windows Vista came out at different times of year, making it difficult to derive broader meaning from the numbers. In addition, the NPD numbers don't include sales to businesses, although the Wall Street Journal quotes an analyst with technology reseller CDW Corp. saying that demand for Windows 7 and new PCs from its customers has been strong.

Microsoft CEO Steve Ballmer told reporters in Tokyo today that Windows 7 sales have been "fantastic."

NPD said in a news release that sales of packaged versions of Windows 7 in the U.S. were 234 percent higher than those of Windows Vista in the initial days after launch. Revenue was also up, although the difference there was smaller, about 82 percent, because of overall discounting and a lack of promotional activity for Windows 7 Ultimate.

The effect on PC sales was more mixed, making Windows 7's long-term impact on the hardware market difficult to predict.

"PC sales growth (following Windows 7's launch) was higher than any week during the high volume back-to-school third quarter, but wasn’t as strong as growth during the Vista launch," NPD said in the release. "Total Windows PCs, however, were up 49 percent in units year-over-year and were up 95 percent over the week prior to launch. But, PC sales growth during the Vista launch was stronger, soaring 68 percent over the prior year’s sales and 170 percent over the week preceding the launch."

Previously: Full coverage of the Windows 7 launch