Okay, so that title is a bit misleading. Anyway, this post hopes to provide some meaningful answers to the question: what would the world be like if the overwhelmingly dominant operating system was secure in ways that Windows is not. For the purposes of this discussion, I'm defining "secure" by several criteria:
1. All users run as limited users - they can't do administrative tasks or screw with the OS without explicitly logging on as admin or running a program as administrator (e.g. Windows run as or Unix sudo)
2. The system is fully isolating with respect to users - one user may not access another user's data without explicit permission
3. There are no privilege escalation exploits in the OS - tricks that limited users could use to gain administrator privilege without having to enter the administrator password
4. There are no remote exploits in the OS itself - in the kernel, standard drivers, basic services, etc.
So, we have this idealized, nonexistent operating system; let's call it Qunix. How exactly, then, would the world look if Qunix had 95% market share? Would this be, as the average Slashdotter seems to believe, a secure and malware-free utopia, where nobody knows what viruses, worms, spyware, or security breaches are, because they don't exist?
The answer, actually, is somewhat depressing: the world would look pretty similar to how it looks right now. Malware and security breaches would still be prominent, the security industry (anti-malware products) would still be big business, and the black hat industry would have similar job security. Granted, the nature of malware would be different, but that would not make it any less prolific or dangerous.
Ultimately, those four criteria I specified have one intended goal: to put everything the user does in a sandbox, where it can't harm the OS or other users (this was how Windows NT was originally envisioned, but time has proved that hope misplaced). Let's assume, for the moment, that these measures achieve that goal (we'll come back to why they don't, later). With this assumption, it becomes impossible for a piece of malware (or a hacker exploiting a buffer overflow, or some such) to invade the kernel, either to destroy the system or to merely hide its existence from the user and malware scanners (a rootkit, in other words).
Unfortunately, while there's no denying that this would make the lives of evil-doers harder, this is anything but the doom of malware/security breaches. Even without the ability to harm the OS itself, a piece of malware could still damage that user's data, and data is often more valuable than the computer it resides on.
Furthermore, the ability to invade the kernel is no requirement for a virile piece of malware. While hiding is more difficult, creating a virus/worm/etc. that runs entirely in user mode is completely viable. Macro viruses, worms that spread through chat programs, and old-fashioned viruses that spread from a disk/e-mail to the computer and back would still be viable and common (although, amusingly, Windows is more resistant to this last type of virus than Linux). There would still inevitably be security holes in third party applications allowing an attacker to get a foothold in the computer and execute code under the user's privileges, and the user could still get (their data) owned, without the attacker ever invading the kernel.
Thus, the necessity of anti-malware products would remain. Now, it would be reasonable to assume that anti-malware products would run with administrative privileges. However, this advantage of privilege would only make life more difficult for malware authors. While it would make it impossible to completely hide from a scanner running at higher privilege, there are many ways of obfuscating, evolving, and encrypting a piece of malware such that it is not readily recognizable by a malware scanner.
Clearly this could be overcome by the malware scanner being updated to respond to a new threat... but that's exactly how the world works right now: anti-malware programs must be kept up to date, or they will not be able to protect against everything that has been analyzed (not to mention the time between when a piece of malware is released into the wild and protection is added to anti-malware products). Consequently, malware analysis labs would still be working frantically, and companies would still have support contracts with anti-malware companies to keep their computers perpetually updated with the latest malware protection.
Now, let's make one final invalid assumption, for the sake of argument: through a combination of various methods, such as security cookies, data execution prevention, and other manner of code hardening, that it's impossible for an attacker to penetrate an application running on the computer (e.g. code injection into a web server, an office application executing code in a document, etc.). That leaves one final mode of attack, one which has been used for decades with incredible success, and one which all of the aforementioned measures combined can't stop: PEBKAC; that is, user naivety.
Even if you could stop all remote and automated methods of invading a system, it will always be trivial to trick the user into running something that is actually malware. This fact nullifies every one of the defense measures proposed previously. Even if a user cannot be attacked other ways, an executed program could wipe all their data. Even if a user only runs as an administrator to install new programs/drivers and perform administrative tasks, an executed "installer" could wipe the data of all other users, and an installed "driver" could install a rootkit for future or immediate use. Similarly, even an air-gapped computer (one which has no network connection at all) still remains susceptible to infection (remember, viruses were rampant on air-gapped computers long before networks or the internet entered the average home/business).
To give you an idea how easily malware can spread relying only on tricking users into manually running it, you only need to take a brief look at the Storm worm. While this worm has been revised and updated extensively over its life, it began as a humble executable that was e-mailed to people; when run, it infected the computer. This worm is now considered to compose the largest botnet in history.