should be as simple as right clicking on your current google chrome icon, click option on the drop down menu, under shortcut tab on the box that appears there should be a button for change icon. click this then navigate to where the icons saved to.
I'm not to sure what the process would be for Vista or windows 7 as i use XP but you can google "how to change icons on vista" and you'll find more than enough help on it. hope that helps sorry i cant help much more
I read somewhere that windows 7 still uses .ico so i would assume vista also does and there included in the pack. The way i do it is right click on the already existing google chrome shortcut on your desktop then click properties and there should be an option to "change icon" you'll just have to navigate to the new icon you want. thats how i do it on XP anyway lol