I have written an article elaborating the steps for installing the SQLExpress 2008 with the Default Instance. There is a step wise screen shot representation which shows how to install the true default instance.
As the deceptive default instance option of the install actually installs a named instance, many existing software written using the default instance in SQLExpress 2005 will not work anymore and so this is important to be done.
I had to migrate some data to MySQL and the migration assistant could not handle the named instance so I needed this workround. As I had to search a lot to find how to do it, I thought possibly it would be fine to post them as a tutorial for others to follow.