I have made a Raspberry Pi icon and folder icon for my Windows 7 PC. Both files are transparent 256x256 ico files and have a bit of drop shadow added so they stand off your desktop or folder.
They can be downloaded by following the links below. Note when downloaded icons will appear with a transparent background not black as shown when you click on the link. Will work with Vista and Win 7.
Pi icon http://sdrv.ms/Oaf4Vy
Pi folder icon http://sdrv.ms/PLEhTE