From Jon DeVaan (source = E7 blog):
One important thing to know is that UAC is not a security boundary. UAC helps people be more secure, but it is not a cure all. UAC helps most by being the prompt before software is installed. This part of UAC is in full force when the “Notify me only when…” setting is used. UAC also prompts for other system wide changes that require administrator privileges which, considered in the abstract, would seem to be an effective counter-measure to malware after it is running, but the practical experience is that its effect is limited. For example, clever malware will avoid operations that require elevation. There are other human behavior factors which were discussed in our earlier blog posts (post #1 and post #2).
UAC also helps software developers improve their programs to run without requiring administrator privileges. The most effective way to secure a system against malware is to run with standard user privileges. As more software works well without administrator privileges, more people will run as standard user. We expect that anyone responsible for a set of Windows 7 machines (such as IT Administrators or the family helpdesk worker (like me!)) will administer them to use standard user accounts. The recent feedback has noted explicitly that running as standard user works well. Administrators also have Group Policy at their disposal to enforce the UAC setting to “Always Notify” if they choose to manage their machines with administrator accounts instead of standard user accounts.