Okay, I found the site you're talking about I think.
http://www.hack.channel.or.jp/flash/indexhigh.html
It looks a lot like just plain old windows really. Desktop icons, Start menu, that's about it.
Sure, the icons are animated, that's pretty cool, but otherwise it seems like it would be simply a matter of making a wallpaper like that one, and making a black button labeled start which triggers a popup, and some black boxes next to that that do nothing...