Although above quotes do work, unfortunately that is not always the case
For example outlook express to windows mail (newer version of outlook express)
If your mail was in outlook express, the easiest way to move it across, would be to hit TOOLS, OPTIONS, MAINTENANCE and then choose STORE FOLDER (this is where all your messages/folders would be stored) copy the text in the box then pop it into START/RUN, then copy everything from that location into a folder and give it a name then transfer it to your vista unit
Then in vista using windows mail you can then import your messages/folders by doing FILE, IMPORT and then selecting the approriate menu (although cannot remember off by hand) then navigating windows mail to the store location that you have copied from XP.
Bare in mind this is only for messages, not mail server settings etc
Job done