You need to get the pass of the admin account in order to give admin rights to your user.
Well, not admin rights in the old fashioned way, but admin rights adding your user not to the admin group, but to every directive that is useful for you... that way its harder to detect because most people only check for the group the user is in to determine if it has admin rights or not.
So you first need to install a good old keylogger to get the admin pass of the account. But as you can't install a thing on the computer, you need to replace an exe file that is executed on system start with the keylogger exe file...
or...
Use a program like l0pthcrack to get the pass. but thats a bit more complicated as you need the sam, and a restricted user might not have access to it...
Also, beware of auditing... Windows has auditing tools that register most user activities into a log... It is, by default, enabled to log some stuff. But it can be set to audit more events, like password changes, login logout times, etc...
Now, going back to topic....
To delete your accounts, we need the name of 1 char per account at least. so we can locate your accounts and delete them...
(If you find any mistake in my English, please let me know via a PM)
(Really, I speak terrible English, so please, tell me if I make mistakes. I'd like to improve it a bit )