Create a new user / pass combo
get into your mySQL db, and find the table mdl_user
Copy the encrypted password "x88asd89w34nkasdf098df98h234" or such from the very last user in the table Foo to the password for the very first entry in the table (Admin), and paste over it. Then change the username field for the very first entry to Foo.
So now you've created a username/password combo that you know (Foo/Bar), and you've pasted it over your lost username/password. You should then be able to gain admin access your site as user: Foo / pass: Bar
If that doesn't work for you, just create new tables in your database:
Change the table prefix on line 55 or so of config.php from "mdl_" to "mdl_131_"
Let me know if you need help with your mySQL.