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Of the 50,000 enterprise users surveyed by the Cambridge, Mass. analyst firm, 87.1% were still running Windows XP at the end of June, compared to 8.8% for Vista. According to author Thomas Mendel, that implies that the majority of PCs upgraded to Vista were those running older versions of Windows, such as Windows 2000 or 98.

"Vista is 'new Coke,'" Mendel wrote, comparing Microsoft's flagship OS to the ill-fated soft drink. Enterprises still on the fence about Vista would be wise, he said, to "consider following the lead of Microsoft's important partner Intel and re-evaluating the case of Vista."

Mendel's comments undercut the momentum for Vista claimed by Microsoft, which says it has sold 180 million licenses for its 18-month-old operating system to PC makers and end users.

Vista still has double the share of Macs among big businesses, however. The share of Macs grew from to 4.5% in June from 3.7% in January 2008. 80% of those are Intel-based Macs.

Linux's share of desktops, meanwhile, fell significantly, according to Forrester, to 0.5% in June from 1.8% in January.

As a result, enterprise application developers only need to "develop exclusively for Windows XP and Vista. Forget about Macs unless you're aiming at a specific business vertical where Mac use is prevalent."

Forrester's study examined the Web browser as well as the desktop environments of the 50,000 users, spread out among 2,300 companies. It found that 19.4% of enterprise users are using FireFox, up from 16.8% at the beginning of the year. Meanwhile, Microsoft Internet Explorer's (IE's) share only slipped slightly, from 79.1% in January to 77.6% at the end of June.

"At least make sure that applications work on Firefox as well as IE -- this is a must," Mendel wrote.

Apple Inc.'s Safari owns only a small slice of the market -- 2.4%, according to Forrester.