Despite the fact that it's now been on the market for more than 18 months and has been upgraded with a full service pack, Microsoft's Windows Vista operating system is, in the eyes of at least one major government agency, still too much of an unknown quantity to be used even for routine purposes.

The Federal Aviation Administration plans to bar contractors who administer the air traffic controllers exam from using Windows Vista-based PCs. Test takers must take the exam on computers running the older, Windows XP operating system -- which was officially retired by Microsoft last month. Contractors can also provide ATC students with PCs running Windows 2000 to take the test.

The FAA has ruled out other operating systems as well. "Windows XP or Windows 2000 operating systems are acceptable for use. Microsoft Vista, Apple OS, Linux, Unix and other operating systems are not acceptable now, but may be acceptable with later versions of the test battery," the FAA says in a draft version of a statement of work for the testing program, obtained by InformationWeek.

While it's not surprising that the FAA plans to prohibit the use of the non-Microsoft operating systems -- given that most of them are seldom used in business environments -- the agency's plan to bar Vista from the testing environment provides more evidence that organizations that in the past have marched steadily along Microsoft's technology upgrade path are now stalled on XP and have little use for Vista.

Even Microsoft's close partner Intel has decided to stick with Windows XP for internal, corporate use, possibly until Microsoft ships Windows 7 in 2010.

Beyond application incompatibilities, cost is another reason why businesses and government agencies are rejecting Vista. The FAA's work statement, for instance, notes that the contractor can provide PCs with as little as 128 MB of system memory for the tests -- not enough to run even the most basic edition of Vista.

Faced with declining Windows sales, Microsoft is fighting back. On Tuesday, the company launched the online Windows Vista Compatibility Center -- a Web site meant to show the vast array of hardware and software that will work with Vista. The site, however, was unavailable as of Thursday.

A Microsoft spokeswoman said in an e-mail that the site would be online "as quickly as possible," but declined to provide further information.