Hello pkdownunder, and welcome to Vista Forums.
You can store the (ico. and png. files) where you like in Vista for use, but without a 3rd party program, you will only be able to change the icons for folders or shortcuts.
Folders: Folder Icon Change Shortcuts: Shortcut Folder or File Icon Change 3rd Party Programs to try: (not free, but trial versions)
A) IconPackager -
Stardock.com - Your Edge In Software
B) Vista Manager -
Tweak and Optimize your Windows Vista - Vista Manager
C) and others out there
Hope this helps,
Shawn