If someone uses the administrator Windows account for everyday work while having the default UAC settings they will be prompted by UAC whenever an application other than certain predefined system apps requires elevated permissions. From the security point of view this is equivalent to no protections against malware getting elevated permissions:
the Notify only when apps try to change settings option can be subverted by any app simply by injecting a thread into Explorer and doing its dirty work there. Since Explorer is a program that the setting allows to elevate silently, this lets you perform a silent elevation from any thread that has thread injection rights into Explorer (which is pretty much any program running at medium integrity level or higher).
One may, therefore, set the UAC slider to the highest setting ('Always notify'). However, not even such a setting is recommended. As exemplified in a recent discussion in the 'The DMZ' chatroom, many people will recommend to use a standard Windows account for everyday work. If elevated permissions are needed, a UAC prompt will ask for the password of the administrator account and only then allow a process to run, but this time under a different account (the administrator account, rather than the standard account).
How does this protect against malware getting elevated privileges more than using the administrator account with the highest UAC settings?
I can see two possible options and I don't have enough knowledge to say which - if any - is correct:
Under the administrator account with highest UAC setting malware can inject arbitrary code into Explorer. Unlike in case with the default UAC settings this malware won't get elevated permissions immediately, but will still get them at the first time the user elevates Explorer, for example because they need to copy a file to a protected location. If, instead of using the administrator account, the user is logged in a standard account the same thing will happen: malicious code will be run under the administrator account with elevated permissions the moment the user elevates Explorer, with the only difference being that the user will have to enter a password into the UAC prompt.
As suggested in the chatroom elevating Explorer spawns a new process and therefore - provided that Explorer is elevated by UAC and not automatically, as in the default settings - no threats injected by malware can get elevated with Explorer. Again, this would render using the administrator account with highest UAC settings no different from running a standard account.
Is there a 3rd option I overlooked? Does using the standard Windows account offer any security benefits to using the administrator account with highest UAC settings?