My gripe has always been that none of these xfce/gtk/etc. themes are ever actually pixel-perfect, even when they claim to be. Not sure if there is a legal concern there or not.
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.