Here's how to locate the files that contain your e-mail.
Under Windows XP, navigate to:
C:\Documents and Settings\<Account>\Local Settings\Application Data\Identities\{69A174D7-B7C9-4A7C-9A7F-D3ABE2327BC8}\Microsoft\Outlook Express
Replace <Account> with your account name. In the above path, I've used my identity; yours will be different. Note that this path is the default and can be changed.
You should see the following files, possibly others:
Folders.dbx
Inbox.dbx
Offline.dbx
Outbox.dbx
Deleted Items.dbx
If you are using subfolders in Outlook Express to organize your e-mails, you will also see other .dbx files with the names of these subfolders.
Copy all of these files to the new drive. Simply paste them over the existing files on that new drive in the correct location and you'll be up and running.
Note, these files do NOT contain your address book, nor your e-mail account settings, just the e-mails themselves.
To verify that these are, in fact, your files, here's a quick test. With Outlook Express closed, move Inbox.dbx onto the desktop. Then open Outlook Express. Your inbox should now be empty. Close Outlook Express, move the file back, then reopen the program. Your e-mails should be back in your inbox.
On a 9x platform, I believe these files are located here:
C:\WINDOWS\Application Data\Identities\{69A174D7-B7C9-4A7C-9A7F-D3ABE2327BC8}\Microsoft\Outlook Express
or
C:\Windows\Profiles\<User Name>\Application Data\Identities\{69A174D7-B7C9-4A7C-9A7F-D3ABE2327BC8}\Microsoft\Outlook Express
Hope that helps.