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Story: Vista struggles to attract enterprise users
Because there's not much benefit
Unlike home users, it is costly for enterprise to upgrade, it's not just a case of insert the CD and you're done; there's data migration, software from other vendors which might not immidiately be compatible, in some cases user training etc. So it is understandable why they'd take their time.
Vista is more secure but isn't that big an improvement from XP overall, if you factor in hardware requirements and all the other compatibility issues that have plagued Vista, it makes business sense to hold out or even skip Vista and wait for Windows 7!
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