Deleting User Accounts that should not exist

Hi. I did a clean install of Vista Home Premium over XP home. I saved all
the old account details on an external hard drive just in case. I formatted
the XP drive, installed Vista OK and began setting up the user accounts from
scratch.

Imagine my surprise when I wanted to create new user accounts, but with the
old user names, to be told that I could not use characters such as "$%^?" etc
in the user names (which I was not). AFter some digging around, when I got
to parental controls I found that the old account names were listed there
even though I had not yet re-created them! I cannot find reference to them
anywhere else on the C:\ drive (where XP used to be and now Vista lives) nor
in the registry.

So, how come these accounts are listed in parental controls when I haven't
made the accounts yet?
How can I "delete" them so that I can start again?

But spookily, how on earth did they get there?
--
Thanks - Ted