5. Windows XP
Codenamed Whistler, Windows XP is where Microsoft transferred from their blocky grey design to a new cute, cuddly and rounded design.
It came as a shock to many, feeling almost childlike with the sky blue Taskbar and the familiar green grass that adorned the default backdrop - was this really an operating system or a kid's VTech?
As it turned out, there was a lot more under the hood. It was more of a serviceable gaming platform, and saw DirectX come into its own.
XP could run games that are now thought of as classics such as Half Life 2, Star Wars: Knights of the Old Republic and World Of Warcraft. It was a golden age for Microsoft PC gaming.