First month sales of Windows Vista were more than double the sales that Windows XP racked up in twice the time, according to Microsoft.

Microsoft sold more than 20 million Windows Vista licences in its first month, the company said. That compares to the first two months of Windows XP sales, which reached 17 million licences.

The figures didn't impress one expert. "I would be more surprised if that weren't true," said Richard Shim, an analyst with IDC. "Back when XP launched, the cards were really stacked against XP. We were just coming off the bubble bursting and the PC market was in a trough."

In addition, there are a lot more PC users now than when XP launched so Vista sales should naturally be better, he said.

The 20 million Vista licences include software sold to PC makers, upgrades and the full-packaged product sold to retailers from 30 January to 28 February, Microsoft said.

While Microsoft didn't break out Vista sales by version of the product, anecdotally, users appear to prefer the premium version. Dell customers are "overwhelmingly" choosing the premium version of Vista, a Dell spokesman said in the Microsoft statement.

IDC expected that because PC makers have been making more powerful machines in order to support the full Vista experience and they're often selling such PCs without a price premium, Shim said. That's encouraging end users to choose the more robust systems along with the premium version of Vista, he said.

The most important test for Vista, however, will come next year after enterprises have had the time to carefully examine whether or not to upgrade to Vista soon, Shim said. "The question is, does that [sales record] continue and what happens when the commercial market begins to adopt," he said.