Changing Sql Server 2005 password without knowing existing password?

I have a user with no login in my sql server (equiv to sa in power in my database) and I dont know the existing password. How do I force a change of password without knowing the existing password?? sp_password takes the old password as a param - I have never had to do this without knowing the exisiting password before but I am sure lots of people must have done this?Changing Sql Server 2005 password without knowing existing password?
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