I don't remember exactly for Win7, but I think it's the same as Vista. Here are the paths for Vista, and you'll have to enable showing hidden files to see some of them...
C:\Users\YOUR_NAME_HERE\AppData\Local\Thunderbird
C:\Users\YOUR_NAME_HERE\AppData\Roaming\Thunderbird
AppData is the hidden directory. Remember, if you delete these directories, you'll lose any old mail, and old account settings.