Major operating system upgrades, whether XP to Vista or XP to Windows 7, typically take 12 to 18 months, due to all of the app testing, rewriting of custom apps and employee retraining. Due to Vista's and Windows 7's similar codebases, companies that bite the bullet and standardize on Vista today will enjoy a "smoother" upgrade later to Windows 7 compared to the "riskier" move of holding off and move straight from XP to Windows 7, Schuster said.

For the same technical reasons, deploying a new PC with Vista today is cheaper than installing XP on it and then expecting to later move it to Windows 7, she said. "If I make an investment in Vista today, will it pay off when I migrate to Windows 7? The answer is yes, it will pay off," Schuster said.

Companies that stay on XP will miss out on Vista's "modern" features, which include stability -- Service Pack 2 will arrive in the second quarter, Schuster said -- and improved security. "This is the most secure client OS we have ever released," she said.

Schuster's arguments flesh out the same ones made by earlier this month, who said enterprises that from employees running Vista and Windows 7 in their leisure time.

"If you deploy a four or five-year old operating system today, most people will ask their boss why the heck they don't have the stuff they have at home," Ballmer said.