ReactOS® is a free, modern operating system based on the design of Windows® XP/2003. Written completely from scratch, it aims to follow the Windows® architecture designed by Microsoft from the hardware level right through to the application level. This is not a Linux based system, and shares none of the unix architecture.

The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows. This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, such that people accustomed to the familiar user interface of Windows® would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to remove Windows® and install ReactOS without the end user noticing the change.

reactos.org/en/index.html

The deluge of viruses and other nasty wares is due to an OS's popularity because hackers, scammers, and whatevers will go for better Return-On-Investment for their effort. Since Windows is the most popular, and so easy to break into, it is the prime target for virus makers.

I'm not familiar with MacOS but if it shares the same lineage with Linux, then it's admin or root privilege must also NOT be default, whihc really make sit hard for viruses to latch on.

The Mac also is not popular because it is a lesser machine or crappy OS, but because it is so damn expensive. If the price will come down to the level of PCs, I won't be surprised if folks won't rush off to buy their own.

BTW, for the record, the first virus was written for a Mac.