If you were downloading emails to your computer through an email client then they should still be available.
Which email client were you using?
Outlook Express stores your emails in .dbx files in one or more of the following locations:
C:\Documents and Settings\User Name\Application Data\Identities\
C:\Documents and Settings\User Name\Application Data\Microsoft\Outlook Express\
C:\Documents and Settings\All Users\Application Data\Microsoft\Outlook Express\
The Address Book is located in C:\Documents and Settings\User Name\Application Data\Microsoft\Address Book\
Outlook emails are held in a .pst file. The default location for this file is %userprofile%\Local Settings\Application Data\Microsoft\Outlook
The Address book is held in a .pab file.
Vista uses Windows Mail and the .dbx files are held in C:\Users\username\AppData\Local\Microsoft\Windows Mail
In Windows 7 and Vista the default location of the Outlook .pst file is held in C:\Users\username\AppData\Local\Microsoft\Outlook
Thunderbird stores email files in the profile folder which is located in %USERPROFILE%\Application Data\Thunderbird\Profiles\XXXXXXXX.default\Mail\
Inside the profile folder should be a subfolder for each of your mail accounts and another one for "Local Folders". Each of those contains files with the names matching your Thunderbird folder names.
Each Thunderbird mail folder is stored in two files named after this mail folder name:
A File with no extension stores email messages in plain text format, one after another. This file is larger than the MSF index.
A corresponding (smaller) .MSF file contains mail folder indexes and preference data. Thunderbird figures out what part of the message to be displayed, the order of messages within a folder, and the like - all based on the .MSF file information. It is important to note that .MSF files do not contain any email messages and, all in all, are redundant. Thunderbird will rebuild .MSF files as needed if they are missing.
Hope this helps.
