If it's so simple, why isn't everyone doing it ?

Many people are. Big mainframe computers always have non-privileged user accounts. So do operating systems like Linux and MacOS 10, which evolved from Unix. So does Microsoft Vista, which came with many personal computers starting in 2007, and Windows 7.

Windows evolved from hobby computers in the 1980s, which didn't have user accounts. Microsoft added them in Windows NT, but didn't encourage people to use them as so many old programs would not work. But now (since 2006), with everyone worried about security, identity theft and cybercrime, Microsoft made non-privileged user accounts standard in Vista. Unfortunately, manufacturers are still shipping computers with only a single, privileged, account. You need to go into Control Panel and MAKE A USER ACCOUNT.

Why didn't Microsoft do this to start with ?

Good question. Years ago, Bill Gates didn't think the Internet was important, and there were no such things as spam, phishing or botnets. He wanted to make computers easy to use. Then he was stuck. Every time a new version of Windows appeared, millions of users demanded that thousands of old programs continue to work. Most of these either would not work, or were awkward to use, under limited privilege.

Limited Privilege on Other Systems

These screen shots are from a Linux installation, showing the division between administrator and user accounts:

Fedora Core 4 root setup

Fedora Core 4 user setup

These screen shots are from a Windows Vista installation, again showing the division between administrator and user accounts:

Vista root setup

Vista user setup