Microsoft Corp. has had to create a new build of Windows XP Professional for computer makers because the six-year-old operating system's continued popularity has nearly exhausted the supply of product activation keys.

The new build, dubbed SP2c, includes no fixes or feature changes, but was created simply to address the shrinking pool of product keys. XP Pro SP2c, which has been released to manufacturing, will be made available to OEMs and system builders next month, said Microsoft.

"Due to the longevity of Windows XP Professional, it has become necessary to produce more product keys for system builders in order to support the continued availability of Windows XP Professional through the scheduled system builder channel end-of-life (EOL) date," wrote the Microsoft system builder team on its blog Thursday.

Previously, Microsoft has set Windows XP's EOL for retailers and OEMs (original equipment manufacturers) as Jan. 31, 2008, and for small-scale systems builders a year after that.

"SP2c will be released into the System Builder channel in September to provide system builders with a new, extended range of product keys," the system builder team said. The updated build applies only to Windows XP Professional; XP Home, for instance, is not affected.

The move shouldn't come as a surprise; Even Microsoft has predicted continuing strong sales of Windows XP. Last month, the company's chief financial officer said that he tweaked the fiscal year 2008 forecast to account for XP's longevity. Rather than count on an 85/15 split in sales between Vista and XP, said Chris Liddell, Microsoft now expects a 78/22 split, an increase of nearly 50 percent in anticipated XP sales.

Other signs of the not-dead-yet OS's vigor have included retreats by OEMs like Dell Inc. from earlier Vista-only policies. In April, for example, Dell again began offering Windows XP as an option to consumers. It had already done the same thing for small business customers.