Although a "significant" number of corporations are hesitating to move to Windows Vista, businesses should bite the bullet because Microsoft is retiring Windows XP , and there's no guarantee it will deliver a next-generation operating system on time or with compelling features, a research analyst said.

[ Don't want to upgrade to Vista? Join InfoWorld's effort to save Windows XP ]

"Vista is an inevitability, for a number of reasons," said Ben Gray, an analyst at Forrester Research. He then ticked off several, including Windows XP's announced retirement and unsubstantiated talk about Vista's successor, Windows 7.

"They are sort of in a caught between a rock and a hard place situation," said Gray. Administrators may not want to move to Vista, but neither of the alternatives -- the older XP and the not-even-officially-scheduled Windows 7 -- is attractive, he said.

It will become more difficult to stick with Windows XP when top-tier computer makers pull it off their operating system lists on June 30, the date Microsoft has mandated that manufacturers stop offering it on new PCs. The company will also yank XP from retail sales then.

And companies considering skipping Vista altogether by migrating from XP straight to Windows 7 may be punished, Gray added.

"To be blunt, customers know very little about Windows 7," he said, noting that with the exception of a few facts -- the Vista successor will come in both consumer and business editions and versions for both 32- and 64-bit machines -- "everything else is pure rumor and speculation."

Microsoft's poor track record on making release dates and crafting operating systems without discarding major features should make corporate decision-makers take pause, Gray said. "Ironically, one of Microsoft's biggest weaknesses, the unpredictable release schedule of its desktop operating systems, will likely spur adoption of Windows Vista as a result of this lack of faith in Microsoft delivering Windows 7 on time," he said.

"You can't count on Windows 7 being perfect," Gray said.

The aging of XP and the uncertainty of Windows 7 mean businesses really have no choice: They have to move to Vista, whether they like it or not, he noted.

"We get this question daily from clients: Should they continue to deploy XP, which they know and love, or skip Vista entirely for Windows 7?" Gray said. "We're not here to sell Windows XP or Windows Vista or Windows 7, but Vista looks like an inevitability."

In a report he authored, Gray damned Vista with some faint praise, saying that for large businesses, there was "no viable alternative." Companies may talk about non-Windows operating systems -- Apple's Mac OS X and the open-source Linux in particular -- but "they're not looking to swap out thousands of users," Gray said.

"Companies are trying to figure out where these alternatives make the best fit, maybe pilot [Mac OS X or Linux] in small batches, but on the whole, it's almost a check-mark kind of thing."

The lack of a viable alternative to Vista may be a fact, but it doesn't mean that every business likes the idea of upgrading to an operating system that has been dinged by both users and the press. "Vista's problems are driving a lot of companies to rethink everything," said Gray.

The hesitation to bet the bank on Vista was illustrated by a different Forrester report last month. Then, the research firm touted polls that surveyed 50,000 corporate users throughout 2007. The results, said Forrester, showed that Windows XP usage had not budged during the first year of Vista's availability. Instead, the rise in Vista use -- by the end of 2007, 6.3 percent of users were running it -- was matched by a decline in Windows 2000 usage.

"Companies have clearly pulled back on their very aggressive migration plans, in some cases by a couple of quarters, in others by a couple of years," Gray said. "And there's a lot of resentment that Microsoft dominates the [desktop] operating system, which is why people are looking at Mac and Linux."

But businesses should put all that should aside and start their migrations to Vista soon, Gray argued. "There's no question that adoption of Vista has been tempered to date," he said, "but now that [Vista] SP1 is out, that's going to help adoption. It's the official blessing of Vista."