For an easy transfer of thunderbird data from and to windows operating systems use MozBackup. It's created and maintained by Pavel Cvrcek NOT by Mozilla.

This little software can also backup data from Mozilla Firefox, Mozilla Thunderbird, Mozilla Sunbird, Flock, SeaMonkey, Mozilla Suite, Spicebird, Songbird and Netscape profiles.

Just export your data (backup) and then with your new computer/operating system import the data after installing thunderbird.

This is a rather safe alternative from saving your profiles and email by manually backing up the folders in windows xp and windows 7.

If you do have the skills and want to backup by saving the folders without MozBackup

In windows XP --- C:\Documents and Settings\USERNAME\Application Data\Thunderbird

Save the entire Thunderbird folder and transfer it to an USB, burn a cd or send it to any external drives.

In windows 7 --- C:\Users\USERNAME\AppData\Roaming\Thunderbird

Just copy the thunderbird backup folder after \Roaming\

Please post comments or if you run into any problems