The cheapest way for current Windows XP users to get a legal copy of Vista is to buy the upgrade version of Home Basic, which is US$99. But you don't want the cheapest version.

First, the upgrade version will require you to keep your Windows XP CD for years. You do have a Windows XP CD handy, don't you? Second, Home Basic just won't cut it for most people. It lacks the Aero UI and Media Center capabilities. Plus, you can't connect Xbox peripherals to Home Basic. For many, including yours truly, those are the three best reasons to upgrade to Vista in the first place.

Home Premium ($239 for the full and $159 for the upgrade version) is roughly equivalent to Windows XP Home. It's for nontechnical, nonpower users who use their system for lightweight, personal use only. But if you're the kind of person who currently runs Windows XP Pro at home -- and since you're reading Computerworld.com, you probably are -- you'll be happiest with Windows Vista Ultimate. It's got all the fun and goodies of Home Premium, plus the power-user features in the business version of Vista.

Are you sitting down? The full version of Windows Vista Ultimate costs $399. If you have an XP CD, and don't mind the hassle, the upgrade version of Vista Ultimate costs $259. Ouch!

(If you buy the Ultimate Edition, you'll be able to buy additional copies of Vista Home Premium at a cost of $49.99. For technical users, the ideal scenario for many will be Ultimate for you and Home Premium for the spouse and kids.)