From ProIcons.com you can freely download 256 x 256 PNG icons for Windows Vista and Windows 7. Is a collection of free icon sets of over 2500 high quality.
Windows 8 Metro UI Icon Set
The COMPLETED release of Windows 8 Tiled Metro UI inspired icon set. This set currently includes 436 Dock Icons for today's top applications, internet browsers, Operating Systems & Manufacturers and more.Download