Enterprises on Windows XP, however, 'can be a little more leisurely' and begin planning for a three-month pilot of Windows Vista in the first 12 to 15 months after it ships.

Gartner also said that companies considering a one-year extension to their Enterprise Agreement (EA) or Software Assurance (SA) contracts during the next six months in order to get Windows Vista should negotiate a clause that guarantees such an upgrade.

'Understand that Windows Vista Enterprise can be installed only on PCs you own; after SA expires, Windows Vista Enterprise cannot be installed on PCs you buy to replace ones formerly covered by SA,' Gartner wrote.