Here is a beautiful SevenVG R2 theme for Windows XP users by Vishal Gupta. This theme will give a look of Windows 7 to Windows XP OS. SevenVG R2 is an update over previous SevenVG Refresh theme.
Enigma VS is a very basic mod of the default Aero VS in Windows Vista. Enigma VS is made by Solarisphere. The author haven't modded any of the windows, the start menu, or anything else.
Here is a beautiful theme Aero Clear 2 for Windows Vista made by jjoul. This theme is updated version of Aero Clear 2 theme by neodesktop.
Microsoft is offering free E-book Windows Vista Resource Kit to celebrate their 25th Anniversary. This e-book helps you to take your skills and knowledge to the next level. Windows Vista Resource Kit, Second Edition is written by Mitch Tulloch, Tony Northrup, and Jerry Honeycutt with the Windows Vista Team.