Change ownership - Vista Business edition

I'm running Vista Business edition on a laptop I just purchased secondhand.
When I start up I have two option to start as - I think is is "Account
Controller" (I think this is the same as administrator) or as "User".

User, is the name that I think the previous owner changed the original user
name to when he deleted all files etc off the laptop, I say this because if
I go to the command prompt is opens as C:\users\mike_d - even though I am
logged in as "user".

In Explorer, documents and setting \ users there isn't a name 'user', just a
folder mike_d however in the admincentre there is only the listed
user....who is user! (I hope you're keeping pace!).

User and mike_d are the same, it's the same account because if I save a
favourite webiste it appears in my (i.e. user) settings and if I open
explorer and work my way to mike_d's folder it's in there too.

How do I get rid of mike_d? when his account is not listed in the admin
control. I can't delect the mike_d folder or change it's name

--
Ron