ed6040 is almost correct, but actually it's much easier. Don't create any user accounts or do anything else after initial Thunderbird installation in Windows 7, except this.
In your original system (Win XP or whatever) go to "Documents and Settings\application data" and copy the entire Thunderbird directory to somewhere you'll be able to read in Win 7 (e.g. a memory stick, a CD etc.) or if you have more than one HD, you can use the original data directly, as I did.
Next, in Win 7 paste the entire Thunderbird directory into "..\users\computer name)\AppData\Roaming". Job done.
If you now load Thunderbird you'll find everything exactly as it was in your original installation.