There was only one account, and it has been in a situation that there was only normal user account in My PC.
Can I make administrative account newly or normal user to Administrator?
If this fails to work, you could download Petter Nordahl-Hagen's Offline NT Password & Registry Editor as recommended on petri.co.il. Burn it to CD, boot to it and it will allow you to either promote a current user to a local administrator, reset the local administrator password and enable the account if all other suggestions fail. <-- this is free.
EDIT - hmm... I never realised that, the tool above also has the ability to 'Promote a user to administrator'. If you want to see a video/tutorial of it in action, check here.
So, there must have been an administrator account at some time. I presume that you were on this profile, and changed the account to normal. I think that Windows would give an error message, or just not let you do that [Dave].
There must be an administrator account somewhere. Either the user is still the administrator, or the administrator is somewhere else. It could be hidden from the log on screen.
Let us know how correct this is so far. Maybe we can help with some more informaation.
The built-in Administrator account cannot be deleted, but it's normally disabled on Vista. Also, as far as I know, Vista doesn't let you delete the last Administrator account, so your description of the facts is quite puzzling. Let's first try a couple of long shots.
First long shot: Try to use Computer Management (right-click the Computer icon and choose Manage). See Users under Local users and groups. Try to see if you can create a new user or can add your user to the administrators group.
Second long shot: Under Administrative Tools (if not present in Start menu then it may also be in Control Panel), choose Local Security Policy. Select Local Policies and then Security Options and double-click "Accounts: Administrator account status" (which should have disabled as status). Try setting it to Enabled and OK.
If none of the above worked, you will need to enable the built-in Administrator account and reset its password (if any was specified). For example, "Active Password Changer", personal license $49.95, is advertised as a solution, so you may take a look at the downloadable demo. There might exist freeware that does the same which I don't know about.