I would like to comment on my own previous comment.
After having thought some, I've concluded that an irresponsible user is going to have problems even if the root/administrator password is required to install new software. Many people I know who have Windows will get these popups telling them their computer is infected or they need a registry scan or something of that nature. These things are known vectors for viruses and other malware. I tell people not to click on that stuff, but their thinking seems to be "What could possibly go wrong?" This is the case even with people who have had system-destroying virus infections in the past. Norton (or whoever) can never catch everything, even if the definitions are up-to-date, since the authors of viruses, etc. are always one step ahead of them. From what I've seen of irresponsible computer users, an administrator password is likely to be something easy to remember, such as the owner's dog's name. Linux will even let you do this, though if you set the password from the command line you will get a warning that it's a weak password. So, if a Windows user's administrator password is "spot" and a popup appears saying that the machine is infected, and the user clicks "OK," the chances are high that the irresponsible user when prompted by the system for the administrator password will just type "spot" and let in the virus, trojan, or whatever. Furthermore, a lot of Windows software that's "free" as in gratis comes loaded with spyware, and people generally don't do the research they need to do to find out what contains spyware and what doesn't. So of course in those cases (Kazaa or whatever) people will generally give the admin password just because they want to download songs or whatever.
So, I take back what I wrote earlier. Having Windows require the administrator password probably wouldn't make most systems more secure.
The real answer, as GrannyGeek and others have already said, is more responsibility on the part of users. I'm a generally optimistic person, but on this matter I'm pessimistic. If, in the future, children are taught in school the importance of computer security, things might change. With the current users on the Internet, I don't see it happening.
I suppose it's natural that we Linux users, along with *BSD users and other *nix users (I don't include the Mac but that's a different story) are more security conscious than most computer users, just because we tend to be more computer savvy. This doesn't make us "better" than those who are not as savvy, just more knowledgeable in this particular area.
Since this thread hasn't been Godwinned yet, I'd like to take the prize:Hitler's Cat