Until the release of Windows XP, skinning and Windows customization in general was kind of a niche thing. But with Windows XP, Microsoft made the OS much friendlier to skinning. Not only does it come with a visual style engine based on the WindowBlinds design that allows WindowBlinds visual styles to run much smoother, but the robustness of the OS in general has allowed users to experiment with customization safely.
We took a poll on WinCustomize as to what OS people are using and the results pretty much mirror the web logs:
80% of users are were running Windows 2000 or Windows XP (with most of them being Windows XP). The other 20% were mostly running either Windows 98 or Windows ME. The consensus seems pretty clear, those who customize their PCs tend to find Windows XP as the best OS to do it on.