Green, the final safeguard against a virus is always the user. If properly maintained and protected, a Windows PC can be kept virus-free rather indefinitely.
Those who mainly get hit by viruses unfortunately tend to be the "go away, stupid error message" crowd who, when faced with an UAC warning, are likely to just click whatever button they think will make it go away the fastest. That's usually "Allow".
Furthermore, a great deal of viruses are distributed as "trojan horses"; the malware is embedded in a program which the user really wants to run, such as a game (extremely likely if your favourite on-line game retailer has a pirate ship for a logo). The (legitimate) installer will trigger UAC, which the user will allow, passing control over to the malware within. Anti-virus can sometimes protect against this; UAC cannot.
Running as a true limited user account is unfortunately still not an option for many users. The only place where you'll really see it is in locked-down office environments where all PC maintenance is handled by the IT dept.