On Outlook Express on XP.
Click Tools > Options > Maintainence, then click on Store Folder
Copy the entire string. On this PC, it looks like
C:\Documents and Settings\Resource\Local Settings\Application Data\Identities\{592E37E7-A220-4F6A-9E20-387B3DEB8E79}\Microsoft\Outlook Express
Click Start > Run then paste in the string and hit OK.
It will open up the exact folder, copy all the files in there to your new PC.
On the new PC, setup your Outlook Express, then BEFORE you check for new mail, do the exact same thing again, but this time, when you get the Store Folder open, just move the files you copied to this folder. Done.