By comparison, in its first two months, Windows XP sold 17 million copies, Microsoft said.
"We are encouraged to see such a positive consumer response to Windows Vista right out of the gate," Corporate Vice President Bill Veghte said in a statement Monday. "While it's very early in the product lifecycle, we are setting a foundation for Windows Vista to become the fastest-adopted version of Windows ever."
Of course, the PC market has grown substantially since XP hit store shelves. In 2001, worldwide PC shipments totaled 136 million units, while last year the industry shipped 227 million computers, according to IDC.
And Microsoft's figures include not only boxed copy sales and those included on new PCs, but also people who bought Windows XP during the holiday season and have applied for theirsince the .
In an interview, Windows marketing director Bill Mannion said that the upgrade program did help the sales figures, but said it wasn't the driving factor. "It's boosting the overall number, but it's certainly not the core component of the 20 million," he said.
PC makers also say that they are encouraged by early results for Windows Vista.
"Overall we've seen a pretty good reaction to the release of Vista," said Kenneth Walker, chief technologist at PC maker Gateway.
Both Microsoft and the PC makers also say they are seeing a shift to higher-end versions of Vista. When XP made its debut in 2001, it came in two main flavors--Home and Professional. The company eventually added the Tablet PC and Media Center editions, and over time, Media Center became the dominant version on retail shelves. Vista comes in six flavors--Home Basic, Home Premium, Business, Enterprise and Ultimate, as well as a Starter Edition only sold on new PCs in emerging markets.
Mannion said that Microsoft is even seeing better-than-expected sales of the pricey Ultimate edition. "We have relatively modest expectations for Ultimate, but it's exceeding that on both new PCs and the packaged product."
Walker said that Gateway has seen more customers on its Web site choosing the Ultimate edition than it initially expected. Customers who go to the Web often buy high-end machines, and those buyers may want to try to "future-proof" their PC by opting for the most full-featured version they can get.
He likens it to car buyers who buy more horsepower than they need. "How many people buy the V8 instead of the V6?" Walker said. Or how many go with the optional towing package, "even though they have nothing to tow?"
Hewlett-Packard, which sells most of its PCs through retail stores, said it is has seen "not much interest" in the Ultimate version thus far. The company said it has seen consumers opt for PCs with more memory as well as machines with Windows Vista Home Premium.
Bruce Greenwood, vice president of notebooks and North American channel sales for HP, said that with both laptops and desktops, HP is seeing a shift away from the lowest memory systems and those with Vista Home Basic toward machines with 1GB or more of memory.
Although Microsoft is counting in its sales totals those who bought XP machines late last year and have applied for a free "Express Upgrade" to Vista, most PC makers have only this month started to ship the copies. The program has been a source of considerable frustration for many buyers who have had trouble registering and getting approved for their upgrade.
Mannion said Microsoft hopes such problems are largely a thing of the past.
"That appears to be behind us now," Mannion said. "We understand manufacturers to be in full shipment mode."