In the 3.x era, fully replacing the windows shell was easy and there were lots of options. These days, the windows taskbar+start button is just such an efficient use of space and resources that few alternates exist.

There are plenty of way to customize the appearance, though. Stardock has lots of products that change the look and feel of Windows. Start10/11 to alter the start menu behavior. Windowsblinds to skin the overall look and feel of windows themselves. Then there are other launchers that sit (mostly) on top of the existing MS look such as Launchy or RocketDock.