User Accounts and/or undeletable file?

Hi, not sure if I am posting in the right spot, so please excuse me if not.

I have a laptop from my employer that I want to return as my contract is ending. However I am keen to ensure that I delete all my personal info as I also have work for another contract on it which is quite commercially sensitive (unfortunately I had a period when income would not allow me to have a second machine to keep everything separate). Naturally I have transferred everything I want to another machine and so that is safe.

I have deleted everything I want to from the (dell) laptop, and used CC cleaner to wipe the free space, but I note that there is a file still left in one folder with a name that refers to the other company. If the first company sees that name they may think I have leaked info to a competitor and not want to offer me another contract later this year.
I have tried all I can to delete the file but I get an error message saying it can’t be located (0 bytes) which I assume is because I have already deleted it. I have tried logging off / on 10 times and tried to find the file but of course it is not actually there.
I have two questions, how can I delete this “ghost” file, and / or how can I delete my User Account so that when I return the machine, nothing can be recovered?
Of course because it’s a company machine, although I have admin rights as I need to be able to install programmes etc; I may not have Full rights. I have tried to add another user so I can log on as that user to remove my existing account, but I get an error message about Trust accounts.
Any help appreciated.

Errr, but that will remove the Windows installation also wont it and because these days the machine does not ship with a CD, I am likely to get hassel and charged for a new copy...

I would be happy to return it to factory settings, but again I dont have a cd

Not too sure what you mean by a LiveCD.

I downloaded a copy of a w7 repair disc?