How do I set the account "Run as administrator" uses? It doesn't seem to just use the one called "Administrator" which I have created (this is a work computer that I didn't set up).
If you are logged in with an account with admin privileges, then obviously it uses your current account.
If you are logged in with a standard user account, it should prompt you with a list of admin accounts.
Ok so from some other answers it seems that "Run as Administrator" actually runs with the current account, but gives it the full permissions of the current acount rather than a reduced set.
There was another useful one but I lost it. I believe the reason it doesn't prompt me for an account is because the UAC settings have been changed (I haven't checked but that seems the likeliest cause).
"Run as Administrator" runs as the account you are signed in as, I do not believe Windows has such a feature to change how this feature works.