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While customers have run into trouble before when upgrading Windows - consider the XP Service Pack 2 update in 2004 - they should find more support from Microsoft when making the shift to Vista analysts say.

That is because Microsoft is offering a number of application compatibility tools to give customers a hand in not only spotting compatibility issues, but also in categorising applications to get a good view on what software is running. Microsoft provides these capabilities in the new version of its Application Compatibility Toolkit, ACT Version 5, which is shipping with Vista, along with a handful of other enterprise-focused tools and resources.

"You can probably expect more than 90 per cent of your applications to run, but you still need to figure out which ones are going to have issues," says Michael Silver, research vice president, client platforms, at Gartner. "So the first step is knowing what applications you have and then determining which are really critical and deciding which you can get rid of."

Microsoft also has been focused on testing application compatibility throughout the development of Vista, analysts say.

"Vista has the potential to be a bigger issue (than previous Windows updates) because it is a complete code rewrite," says David Bradshaw, a principal analyst at Ovum. "On the other hand, Microsoft has done an awful lot of testing."

Microsoft says that more than 800 applications from more than 150 ISVs were tested against each daily test build of the operating system. "Windows Vista was designed and built with application compatibility at the forefront," Microsoft says on its Windows Vista webpage dedicated to application compatibility.

But while larger software vendors such as Symantec have announced support for Vista, there are many smaller independent software vendors (ISVs) who are not likely to support the operating system for some time, analysts say.

"As you get to smaller ISVs and applications that are vertical applications and more critical to your business, that's where you're going to have problems," says Silver, who adds that organisations running homegrown software will also face some hurdles.

"Probably the biggest issues have to do with user rights and running as a standard user," he says.

Indeed, authentication and security have already sparked concern in compatibility testing during Vista's beta cycles, notably the overhaul of the core Windows log-on architecture that may cause migration issues for those running third-party Windows-based authentication systems such as single sign-on and VPNs.

ISVs have been working through the issues in switching from the old Graphical Identification and Authentication (GINA) model to the new Vista Winlogon Re-Architecture and Credential Provider (CP) model. The GINA is used by vendors such as Checkpoint, Cisco, Citrix, Nortel, Novell, RSA Security and Symantec to write modules that link their authentication technology into the Windows authentication architecture. Microsoft has said there will not be any backward compatibility for GINA.

Users are likely to face testing periods after vendors deliver their new interfaces, and users with any home-grown authentication mechanisms linked to Windows will have to re-write their code from the ground up.

As part of the process, users will have key security infrastructures that straddle two different authentication environments, one for Vista and one for earlier versions of Windows, until migrations are complete. They will also have to support different client-side code and separate interfaces that will present re-training issues, experts say.

So while Microsoft may be launching Vista on 30 November, industry experts say they do not expect to see wide enterprise roll-out of the new operating system until 2008.

"Typical large enterprises will spend 12 to 18 months testing their applications and preparing for the move," Silver says. "We don't see mainstream deployment until 2008."

Tom Henderson, principal researcher for Extreme Labs and a member of the Network World Lab Alliance, agrees that roll-outs should be done bit by bit.

"It's a good idea to follow Microsoft's advice, in terms of setting up group policies to control Vista's behaviour before it's rolled out," he says. "It's wisest to think about piloting varieties of machines grouped by application use, then install required and desired programs to check behaviour settings before rolling things out."

"In no event should someone just go to the corner, buy a machine and put it on their corporate network," he says.

- Network World Senior Editor John Fontana contributed to this report.