Windows 7: Cannot change back my account to administrator

Hello,
My problem is - I CAN'T change back MY USER ACCOUNT from 'STANDARD USER to an 'ADMINISTRATOR'. Now, my account is a 'standard user', and i was UNable to change back MY ACCOUNT to an 'administrator' at the user account .When i try to change 'ASP.NET Machine account' account type,i was UNable to change it to 'Standard User' the note given i ''must assign another user on this computer to has an administrator account before you can change this account type''.how i can do that?.Next,at my user account,i click at the change account type, after that,i choose 'administrator' but it still as a 'standard user', it was UNable to change back my account to 'Administrator'. Only 3 account at this computer.1.My account now as a 'standard user' where are the first account as administrator at this computer,2. the ASP.NET as an 'administrator' when i accidentally change my user account type to 'standard user' and it automatically changed the ASP.NET as an administrator at this computer,the last account is Guest, the Guest account is Off, Only me use this computer. I was ACCIDENTALLY change my user account from 'ADMINISTRATOR' to 'STANDARD USER',after that, ASP.NET Machine account AUTOMATICALLY changed as an administrator at my computer, and now i CANNOT change back my account to 'ADMINISTRATOR', i want to change back my user account from 'standard user' to an 'administrator account'. Can you tell me how to CHANGE BACK my user account to ADMINISTRATOR ? Please . . . .

Start in Safe Mode by pressing F8 on boot. Safe Mode
Then you can login as user "administrator" (try blank password). Now you can change your user type

UPDATE:
Sorry but what I said isn't working!
You have to logon with an account with admin rights to do it. Since this is impossible for you you have to enable the built-in admin account called "administrator". Is has a blank password by default.

For 32bits Windows 7 it's easy, for 64bits Windows 7 more difficult

Built-in Administrator - Enable from WinRE