On your old computer go to-
C:\Documents and Settings\UserName\Local Settings\Application Data\Identities\{2C7D8CD1-00FA-123F-BF86-DD123A20D123}\Microsoft\Outlook Express
The folder name with all the letters and numbers will vary. There may be more than one depending on if there were more than one identity set up.
Copy the entire contents of the Outlook Express folder.
On the new HD, find the same folder path (remember the one will vary) and replace the Oultook Express folder with the old one. Be sure OE is not running when you do this.
If you copy and paste the right folder to the right place, you will see all your emails and still retain any folders etc that you created.
Now, if you have already downloaded main into the new OE that you want to combine with the old, it gets a little more complicated.