You can copy the files, I dunno how to copy the settings though.
Computer 1:
Vista: Copy C:/Users/Your Name to a flash drive.
XP: Copy C:/Documents and Settings/Your Name to a flash drive.
Computer 2.
Vista -> Vista: Copy Your Name to C:/Users.
XP -> XP: Copy Your Name to C:/Documents and Settings.
Vista -> XP: Rename every folder in Your Name to "My (whatever)" and put it into C:/Documents and Settings
XP -> Vista: Remove the "My" in every folder in Your Name and put it in C:/Users
Idk.
Very useful post, good job.