Dbo Owner

We are using SQL Express and I have a problem with dbo.
On my machine if I logon to SQL Express using windows authentication
and create a new database I automatically have db_owner role membership
on the new database.
On a colleagues machine, if he logs onto his SQL Express using windows authentication
and creates a new database he does NOT have db_owner role membership
on the new database.

How come?
I have checked pretty much everythng - windows built in admins, SQL express sysadmin roles
service pack versions but I can not find any difference in setup.
What should I do to his machine/SQL Express setup so he automatically has db_owner role membership
for every newly created database?

Thanks
Charlie