User permission and password related query

Dear Techyv,
I thought I should ask you first I purchased a second hand good computer from a friend who was moving interstate just as a spare computer.
A week or so later my neighbour had a problem with his computer so I loaned him this computer. 4 weeks later he gave it back it had a pop up where I had to log in when starting up the computer.
I tried to remove having to log in to start up the computer by setting up a second administration user so I could cancel the 1st admin user then add a new password for the second admin and clicking remember password allowing me to simply start up with out passwords but some thing went wrong and I ended up with a user with limited permissions now I can use the computer but have no admin rights to add/remove/change any thing I can't even delete the operating system to add a new one.
Is this something you can do to fix & bring it back to a normal – no password with full administration rights.
