You can do it manually by copying the profile folders over, but the easiest is probably MozBackup. Install and run it on the XP system and create a backup file, you can choose where it will be saved.
Copy that file to the Win7 system using a thumbdrive, CD, or a shared folder if you have one. Install Thunderbird on the Win7, and run it once to create the user folders and set their permissions correctly. Then install and run MozBackup on the Win7 and use 'Restore' to copy your saved XP profile to the new Win7 system.
MozBackup is open source freeware, you can download it from CNET.