Have you actually logged in as the administrator? All windows accounts have a user name of "Administrator" built in.
You may find that this account has been de-activated.
Have a try of the following..
Start > type "cmd" > right click and "Run as administrator"
net user administrator /active:yes
you should get a response saying the command was sucessful
next, set the administrator password by doing the following:
net user administrator* (yes, put the asterix)
you will be prompted to enter the password. when you type it in, you won't actually see any of the keystrokes you type, but just hit enter when you are done, you'll have to confirm the password by typing it in again.
Once it says it's sucessfull, log off and then log back in using Username: Administrator, Password: Whatever you set.
Then do whatever changes you need on there. (While you're at it, you can check weather your other user name has full unrestricted access. Some accounts are labelled as an adminstrator but still have some limitations.)
Worth a shot

I may have taken you on a pretty far fetched route though.