I wouldn't even want to guess how many operating systems I've used. Most versions of DOS, most versions of Windows, several versions of Unix, RSX-11M, several proprietary versions of machine language, and a bunch more I can't even remember. And most of them have more similarities than differences.
Going from one version of Windows to another is one of the easiest transitions you can make.
Learning most of Vista (but not all) should take you all of 10 minutes.