More than legal concerns is that theming engines on each DE can be a bit limiting trying to do such things. i.e. for KDE Plasma you'd need to do a fair amount of SVG trickery to reach 100% accuracy respect of the originals.

Plus the icons thing - they're not as easy as people might think.

But this xfce-winxp-tc seems to be really, really close to the original. In those first two screenshots I thought it was just screenshots of the original WinXP.

Also the CSS framework subject of this thread looks really identical to the widgets and stuff of Win98.

But if you want true 100% pixel-perfect windows 98/xp/whatever... Just use them.