Because they were so expensive, people didn't own one, they bought computer time. Universities portioned out CPU time and disk space to hundreds of researchers and students.
Since computer resources were so valuable, the computer makers invented operating systems to manage these resources and keep track of costs. User accounts were restricted to the resources that they were allocated, and could not access disk space or CPU time belonging to someone else, or to the operating system. The first hackers appeared, finding ways to steal computer time and access system files, but that's another story.
Around this time, a company called Digital Equipment (since acquired first by Compaq and then HP) started selling "minicomputers", that were much smaller (about the size of a refrigerator) and cheap enough that a research group could have one all to themselves. Pretty soon people were hooking them up to TV cameras and coffee makers and generally having fun inventing computer networks.
The minicomputer users couldn't be bothered keeping track of who used which minutes of CPU time, and had better things to do with their RAM and CPU than bean-counting. Digital provided a single-user operating system called RT-11, which had some weird commands like PIP (peripheral interchange program) to copy files around.
About this time, researchers at Intel invented the microprocessor, originally a 4-bit CPU. Pretty soon there were 8-bit processors from manufacturers like National Semiconductor, Fairchild and Zilog, and people started building their very own computers - "personal computers". They started out programming them in hexadecimal and storing programs on audio cassette tape, but pretty soon got fed up with that and wanted an operating system and disks. Along came CP/M, looking remarkably like RT-11 with PIP and drive letters, but running on Intel 8080 and Zilog Z80 processors. Like RT-11, this was a single-user operating system with no disk quotas, passwords or accounting. If you wanted to keep your work private, you locked the door. These early PCs didn't have networking or remote access; if you were lucky you had a 300-baud modem that could copy files from one of the big time-sharing machines.
A bit later came 16-bit microprocessors, the IBM PC and Bill Gate's deal to acquire DOS - "Disk Operating System". This was pretty much like CP/M with drive letters and so on, but it was still a single-user system with no concept of security - after all, no-one was doing anything important with it like banking or brain surgery, and you could still just lock the door.
Then people started wanting to connect PCs together with something a bit faster than phone lines. A whole bunch of different networking methods arrived - token ring, Banyan Vines, DECNET, Novell, Ethernet. People loaded stacks of floppy disks into PCs, wired up ISA cards from different manufactuers and soon had whole offices networked. Meanwhile, something called the internetworking protocol started gaining ground over the ISO standard DECNET, and users started buying third-party software to connect DOS PCs to the "internet".
Suddenly the PC, long the butt of jokes about "organizing recipes", sneered at by IT departments as a toy machine for those unable to afford the real thing, became a force to be reckoned with. But it still had no security whatsoever.
Microsoft hired some computer experts who had worked at Digital to create a new, more powerful, operating system that would be more reliable than Windows 95 and could be used for databases and other business functions. This became Windows NT, and it had many sophisticated capabilites such as user access controls and process management. However, these controls were hard to use and were generally hidden from users, while NT was seen as not suitable for home use - apart from the complexity, many programs such as games would not run.
Windows 2000 was targetted mainly at business use again, and non-privileged accounts were awkward to use - to install software, for example, you had to log out completely from the user account, losing anything on the screen, and login as an administrator
With Windows XP, finally it was possible to switch users easily without losing the desktop. But the wording of the user account screens discouraged "limited" accounts, while most systems came pre-installed with a passwordless administrator account.
With Windows Vista, slated for release late in 2006 (release candidates are available now - Nov 2006), Microsoft has finally switched to promoting the "standard account" for normal use. Not before time.