Windows 7 - Single User Security

This has most likely been covered before but I am wondering about admin accounts vs non admin accounts.
I understand that if you have a network you most likely would want to have an admin and a group of users.
But what if you have a single computer with only one user?
Is there any advantage or necessity to have an admin account and a user account ?
Is an admin account necessarily more secure to the outside world? as in protection from hackers, malware, viruses, etc.

With a single pc you should create a standard user account and use it for everything. Only use the admin account when necessary.

is this for external security concerns?

is an admin account actually any different from a standard user account or is it only about permissions?

with the standard user account I am constantly asked for the admin password to do anything like add / remove programs, etc. so it's a pain in that respect.

if it's only about permissions and there is only one user, what's the advantage to not using the admin account as the regular account?

Well not running with administrative power prevents rouge anti-virus software from infecting the entire system when it exploits flash from legitimate websites.

I usually have a two-user setup: an admin (with UAC set to default, not asking for password but only prompting for consent*) and a standard user, who can only use already installed software and who obviously has limited rights.

Once you initially set up your system, you will barely notice UAC really. And for personal experience I can tell taht UAC actually helps preventing annoying things to happen to the PC.
Plus, I find it pretty effective even in its default settings, so you don't even need to deal with its settings.

*as advised here (I know it's for Vista but I find that reason convincing enough to carry on that setting to 7): Understanding and Configuring User Account Control in Windows Vista

If the account, be it (not the real) admin or non-admin, is exploited, security protocols still have to be followed through. On a non-admin, damage may be limited, but the same rigamarole has to be endured - clean and rinse, clean and rinse. With a (not the real) admin, the UAC still exists. The hassle of selecting an account is removed. The duplication of systems files on a single hard drive for more than one account is lessened. And taking the nuclear option (reformat and reload) is less complex in decision-making.

If you're talking about the real admin account ... not the sole account one sets up out of the box ...

I think it depends entirely upon your computer skills, your computer useage and surfing habits, what malware protection you have in place and what methods you have to recover from damage.

As sole user I used to run as admin on my XP system for many years without a single infection and I am currently doing the same on my Windows 7 system with much the same result. It should be noted that I do not play games on this system, I do not use p2p or search for warez and I am careful as to what I install. Other than that I probably surf to as many unknown sites as many do. Like most others I value the attempts by the OS provider to minimise security problems but when it just becomes too much bother for me personally I take control - that is I run as admin.

I would not advocate this unless you do understand the risks and do have good back-ups of your system and data and can recover from any malware - something that I have never had to do.

The only reasons I have ever had to recover (using system images) is from my own mistakes, which I have done many times. I have used a variety of security products over the years but mainly Norton NIS which appears to work, as I have said - no malware. I do tend to use Firefox with Noscript and no saving cache to disk enabled when I am not sure as to the risks of any particular website and I clean up meticulously after surfing - more from a maintenance point than for malware but it tends to help too. I also do not have Java installed or use Adobe Reader but I have had them in the past.

Is it worth the risk - only you can decide.

Very true, in the past, "legitimate" website have been infected by malicious banners and malware in general, and more then likely will suffer attacks in the future. Right now the list of "Legitimate" sites infected in the past include Fox News, Time Magazine, The New York Times, etc., etc.