Microsoft Windows is, by most measurements, the most vulnerable platform to malware, viruses and other nasties. One main reason for this is that Windows is the most popular and most widely used operating system in the world. This makes it the prime target for malware and virus authors.
However, it should be noted that the first versions of Windows weren’t exactly hardened against malware and virus attacks, or other security threats.
Later on, Microsoft became much more concerned about the security of the platform, but by then, the damage had been done, and numerous security holes and malware and virus attack vectors had been discovered.