By Slash Lane

Published: 09:00 AM EST

Sales of Windows PCs grew faster in the United States during the month of November than Macs, which were weighed down by sluggish demand for aging iMac models, according to NPD.

The market research firm, which tracks retail sales, released preliminary data for the second month of the December quarter on Monday, showing Apple's sales were relatively flat year-over-year while sales of systems running the Windows operating system from rival Microsoft corp rose 7 percent.

Overall, sales of desktop systems fell 20 percent; Windows desktops were down 15 percent while sales of Mac desktops slipped an eye-popping 38 percent.

Apple faired better in the notebook department, where a refreshed line of portables helped boost sales 22 percent year-over year during the month of November. Windows notebook sales rose 15 percent.

"For notebooks, there is a little extra value to consumers (to buy Apple)," NPD analyst Stephen Barker told Reuters. "For desktops I'm not so sure. To me the real story is the iMacs need a refresh."

Indeed, AppleInsider this summer cited reliable sources as saying Apple's goal was to round out the 2008 calendar year with slight revisions to the iMac line. The new models were expected to appear around November in an effort to boost sales ahead of the holiday shopping season.

For reasons still unclear, the Mac maker pulled back on the release in early November, issuing a statement to the press saying that its holiday product line was set and no new Mac models would be introduced until the following year.

It's believed that the iMac refresh is now likely to appear at Macworld alongside the first major overhaul to the company's Mac mini line of tiny desktops.