As anticipated, Microsoft Corp. on Wednesday night launched the beta of Windows 7 , posting the preview of the company's next operating system to its developer download services.

The general public will be able to download the beta starting Friday, said CEO Steve Ballmer in his first-ever opening speech at the Consumer Electronics Show in Las Vegas. "We will make the beta available worldwide," Ballmer said. "I encourage you all to get out and download it."

Microsoft made it clear that the beta will be available for a "limited time," and said it will cap the beta after the first 2.5 million downloads.

IT professionals and developers who subscribe to the Microsoft Developer Network (MSDN) or TechNet services, however, get a jump on the public at large; they can grab the beta right away, said Ballmer.

The beta, which Microsoft called "feature complete," requires a PC with a 1GHz processor, 1GB of memory, 16GB of available hard disk space and support for DX9 graphics with 128MB of memory, according to Microsoft, which also warned that the recommendations could change for the final version. The beta only supports an upgrade from Windows Vista Service Pack 1 (SP1).

Microsoft declined to get specific about upgrade paths for the final version of Windows 7, or to spell out how many editions it would produce and what it would charge for each. The beta is "roughly equivalent" to the Ultimate version of Vista , it added.

Both 32- and 64-bit versions of the beta will be available for downloading, but only English, German, Japanese, Arabic and Hindi editions will be posted Friday. Other language versions are expected at the product's launch.

To install the beta, users must have a DVD drive able to burn disk images to a blank disc. The beta, said Microsoft in a follow-up blog it published Wednesday, will be available as an .iso file. It did not spell out the size of the download.

The beta expires on Aug. 1, 2009.

Not surprisingly, Ballmer was bullish on Windows during his speech, even in the face of an economic downturn. "No matter how long this recession lasts, our digital lives will get richer," he said early in the keynote. "[And] Windows will remain at the center of peoples' technological solar systems."

His announcement of Windows 7 availability was no surprise. Expectations that he would take advantage of the CES stage began to build last month, when clues on Microsoft's own Web site pointed to a release no later than Jan. 13 . Less than two weeks ago, copies of a Windows 7 build believed to be the same as the beta leaked to file sharing sites , where they proved extremely popular.