The first is called xpize, and it's a resource replacement package which updates all sorts of icons and graphics all throughout the operating system, making everything look more unified and modern. It doesn't change the visual style - it just updates the Windows 9x-like icons scattered around settings panels and other places in Windows XP to use Windows XP-compliant ones. The screenshots speak for themselves: a lot of effort has gone into this project.
For Windows Vista, there's Vize, which does the same as xpize, but then for Windows Vista. Basically, it address all those issues which at least make me face palm every now and then when using Windows: they've got this brilliant team of icon designers (Vista and 7 icons are mind-blowingly beautiful in my opinion, perfect compromise between photo-realism and usability), but yet, Windows is still riddled with old-style, 9x (or even 3.x!) style icons and bitmaps. Vize addresses these issues for Windows Vista.
And the big surprise is that the tools used to create xpize and Vize are available on Codeplex, called Anolis, licensed under the GPL as open source. Using this tool, anyone can easily create a resource replacement package which will work on any version of Windows - including 64bit.
These are invaluable tools, as Microsoft can be pretty stubborn when it comes to updating its operating system with modern graphics and icons. Windows 7 is a huge stride forward in this regard, but it still drops the ball on a number of occasions, and I'm hoping Sevenize will fix these issues.