Windows 1.x (1985) and Windows 2.x (1987)
In the first two major releases of Windows, application icons only appeared when users minimized a program to the Taskbar at the bottom of the screen (Windows 1.x) or on the desktop (Windows 2.x). These icons were simple black and white illustrations with a size of 32x32 pixels.Windows 3.0 (1990)

Windows 3.1 (1992)
In Windows 3.1, the icons remained fundamentally similar to those in Windows 3.0 but with more detail, albeit still 32x32 pixels and 16 colors. Microsoft's design artists achieved this by using color blending effects in the icons to simulate more color depth and improve shadow effects.Windows 95 (1995)
In Windows 95, many icons underwent graphical renovations, although some remained unchanged from the Windows 3.1 era. Most system icons were 32x32 pixels and 16 colors by default, but the Win32 API at that time had the capability to support icons of 256x256 pixels with 16.7 million colors for the first time. In fact, with the Plus! add-on pack (a registry hack trick), users could activate icons with 65,536 colors (referred to as 'high color' at the time), although not many users utilized this feature extensively.Windows 98 (1998)
Windows 98 introduced default 256-color icons at 32x32 pixels. This marked Microsoft's first provision of many system icons with a larger 48x48 pixel size, ideal for accessibility purposes and use with high-resolution screens (though quite rare at that time). Several icons like My Computer and Recycle Bin received updates, while many inherited icons from Windows 95 and even from Windows 3.1 in some cases.Windows 2000 and Windows Me (2000)
Similar to Windows 98, Windows 2000 comes with 256-color system icons, with options for sizes of 32x32 and 48x48 pixels. Some key icons on the screen have been upgraded once again with more details and color depth. Windows Me used many new icons that look similar to Windows 2000, but also introduced new ones like the new My Computer icon.Windows XP (2001)

Windows Vista (2007)
In Windows Vista, Microsoft introduced the new Aero interface, emphasizing on translucent effects and shadows. For the first time, Windows came with a set of system icons sized at 256x256 pixels. However, the icon set was still incomplete, with smaller icons being automatically resized to fit the overall theme. As a result, Windows Explorer in Vista also allowed for flexible resizing of icons according to the user's personal preference.Similar to XP, many application icons and main utilities in Vista were redesigned with glossy Aero style to approach a modern interface, independent of the resolution of Mac OS X.Windows 7 (2009)
In Windows 7, the system mostly utilized icon sets similar to Vista but made changes to some key icons for programs like Control Panel and Microsoft Paint. Some icons were modified to have a flatter design, beginning to move Windows away from the glossy icons of Vista.Windows 8 (2012) and Windows 8.1 (2013)
Windows 8 received a new user interface design called Metro. Metro introduced a new type of icon called Live Tiles that allows for dynamic information updates on the tiles (similar to widgets) on the Start Screen.In Windows 8, many application icons became simple white outlines of objects or shapes simulated against a uniform background color. Additionally, Windows 8 also included regular desktop icons, with most of them carried over from Windows 7 and earlier versions.Windows 10 (2015)
Upon its release, Windows 10 initially featured the Live Tiles icons interface from Windows 8 while still using desktop icons from both Windows 8 and 7. Windows 10 also included some redesigned desktop icons with sharper edges and smoother color transitions. In 2020, Windows began rolling out new application icons in the Microsoft Store.Today, the icon set of Windows 10 remains a massive jumble of at least 3 or 4 different inherited styles from previous Windows versions.Windows 11 and beyond (2021)
Over the past few years, Microsoft has gradually revealed a completely new unified icon set for Windows 10. Initially introduced with Windows 10X and later planned for release in the upcoming Sun Valley update, these new icons may now potentially debut with Windows 11.Windows 11 seems poised to completely abandon the Metro/Live Tile concept of Windows 8 and 10, with icons possibly featuring more depth and color. Microsoft is aiming for a flat, low-detail interface with subtle color transitions. It's a welcomed change for many users, especially if Microsoft can finally overcome the clutter of icons from multiple Windows generations currently present on Windows 10.Windows 11 is on the horizon, let's wait and see!Source: HowToGeek