Vista - Listed User has no account

I just installed Vista Ultimate on a new Intel E8400 processor based machine and activated it. I originally had installed the software on an AMD powered machine that relied on Nvidia drivers and had far too many problems. Now I have seen a new problem that I can't explain but it either was caused by some careless act on my part or by some other unknown operation. I noticed that my C drive had shrunk by about 18 GB for some reason so I started looking at tree sizes and found that the C:\Users folder was out of range. Then I found that there was a user listed that does not show up when I look at User Accounts in Control Panel. By coincidence or by some stupid error, the name on the account is my First ans Last names just when I entered it as the computer owner. The mystery user folder (attachment 1) looks normal at first; it has the expected custom folders inside and most are empty . Then I clicked on the first folder listed, the hidden folder, AppData (attachment 2) and found it contained about 17.5 GB of data. The contents include a second AppData folder and a duplication of several large folders from a different user account. I opened the second AppData folder (attachment 3) and while it looks normal it is definitely not identical to the one a level above it. I guess I could just go into the tree for this mystery user and just start deleting folders but because of its name I want to be cautious and not trash my whole installation. I hope someone can suggest a cause of my problem and suggest a save way out of it. TIA Cecil Britton