If you can believe it for some reason I still have the Microsoft User Data folder in my Doumments tab with my 2008 Office stuff... and my 2004 version... which will lead shortly in to a bout of document folder re-organization! 😛
I'd search your home folder for "Database", it seems Entourage automatically produces the file and sticks it under
/Users/username/Documents/Microsoft User Data/Office 2008 Identities/Main Identity.
The default size is a mind boggling 21.1MB. So if you find anything bigger it's probably your old database with your emails.
Also under your list of users, check for any odd folders with your old username on it, it's possible (but not likely) that when you changed your user name it created another user account and deactivated your old one...