Welcome to the Seven Forums!
You might want to contact that shop and ask what they did to cause this. Just write down what they say and repeat it to us here. If that is not an option - then we can take some educated guesses.
It sounds like they made your account a standard user. If you are operating as a standard user (which is a good thing), then certain actions will require information about an administrator account.
Let's test something:
Please click on the Start
button (or orb)
Then right mouse click on Computer
Click on Manage
from the context menu.
You might only see one account listed on the next screen:
Just click on the button named Yes without entering a password and let us know what happens.