Assuming that you're using Outlook Express, you can get some idea of what traces can be found by doing this:
Search for *.dbx (Depending upon your operating system, you might have to specify that the search icludes hidden files).
This will bring up Inbox.dbx, Deleted_Items.dbx, etc (i.e. all of the files used by Outlook Express to store your e-mails).
Right-click on one of these files and select 'Open With'. (This usually works best with Inbox, rather than Deleted Items).
Choose Notepad as the application to read the file with. Even if you regularly clear your Inbox and your Deleted Items, it's quite likely that you'll see many mail items which you thought were destroyed months ago.
Chris