Microsoft is extending the deadline for making its supposedly retired Windows XP operating system available to custom PC builders.

Under the plan, system builders will be allowed to take delivery of XP licenses and media through May 30. Previously, Microsoft had announced a Jan. 31 XP cutoff date for system builders, which are typically smaller, build-to-order vendors. The news was first reported Friday by InformationWeek.com sister site ChannelWeb.

It's just the latest in a string of reprieves for XP, which Microsoft may be reluctant to pull from the market given the poor reception that its successor, Windows Vista, has received from business customers. Many are unhappy with Vista's system requirements and lack of compatibility with older software. As a result, few large enterprises have upgraded their computers from XP to Vista.

Microsoft also originally planned to stop distributing Windows XP media to large OEMs, like Dell and Hewlett-Packard, on Jan. 31. But the company announced in October that it would move that deadline to July 31.

If Microsoft is on schedule with Windows 7, that would leave a gap of about just six months between the end of the XP program in most markets and Windows 7's general availability in early 2010. It's a sign that Microsoft has conceded that Vista is a flop in the key corporate marketplace.

Few large companies are eyeing Vista. A recent survey by the United Kingdom's Corporate IT Forum showed that only 4% of businesses in that country are using Windows Vista on workplace systems, while 35% said they were "not yet interested" in Vista. Fifty-eight percent said they were still using Windows XP, which is now 7 years old.