You're right to change operating systems carefully -- it's a big step. Here is a roadmap for the initial stages:
Take an inventory of applications: You want to make sure they're all compatible with Vista, or that suitable accommodations can be made for running them under Vista.
Perform backups: Chalk this up to the "you just never know" rule, but always make a backup.
Investigate and use mass deployment tools: Unless you're just dealing with the smallest client, you can almost always save time and achieve a more consistent, secure, and problem-free installation by using the deployment tools that I've outlined previously on this site.
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