Ian Moulster, a .Net product manager based in the U.K., blogged that the take-up on the free-of-charge beta had been "incredible," and that with the company's self-imposed download and mail-delivered limit fast approaching, the offer would be withdrawn.
"When it's gone, it's really gone," Moulster wrote.
Microsoft confirmed the Friday shut-off in a message posted to the Vista Beta 2 site.
The Redmond, Wash. developer released Beta 2 early this month, and at the time indicated that supplies might be limited. Currently, Vista is scheduled to release in November to corporate customers, and to consumers in January 2007.
Moulster also urged Beta 2 users to activate their copies. "We may be able to provide people who have activated copies with future...stuff," he wrote. "I'm being vague because I need to be, just trust me and make sure you activate."
Moulster's coyness may be undeserved. Previously, Microsoft has said that users must activate their Beta 2 copy in order to receive RC1 (Release Candidate 1) of the OS when it's delivered later this year.
Office 2007 Beta 2, meanwhile, which Microsoft opened for download in late May, continues to remain available.
Windows Vista Beta 2 can be downloaded or ordered from here. Users who have already received a product key -- obtained by registering online for the beta -- will have until July 14 to download the code.