Vista Transformation Pack [Download]
Vista Transformation Pack does all kinds of appearance tweaks, and makes Windows XP look like Vista. From icons, transparency to new sounds, this pack does it all to make your XP look like Windows Vista.
Mac OS Transformation Pack [Download]
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Transforms XP to look like Mac OS X 10.5. Some important components like Windows Explorer and other shell files are patched to mimic Mac OS X appearance. Work only with Windows XP SP2 (English version).
Again, make your windows look like Mac OS X. However, it's much more comprehensive than the above one. This enhancement utility does everything from registry tweaks, visual styles, sounds, icons to imitate the appearance of Mac OS X. See other screenshots here. If you're using iTunes then make sure to deselect 'iTunes Multi-Pugin' option (under 'Software') in the installation wizard.
Fedora Transformation Pack [Download]
This tool is intended for making XP resemble the Fedora Linux Distro. Though the utility just packs a simple visual style, it does include icons, screensavers and wallpapers, and other Fedora-like skins for programs.
Ubuntu Transformation Pack [Download]
This pack contains visual styles, icons and other patches that can give the brownish Ubuntu look to Windows XP. If you just can't get Ubuntu working on your PC, but still love its looks, you can download this pack. [via]