Icon Explorer is a free and portable tool which makes it easy to locate icons on a Windows system, then save them as ICO or BMP files.
For a simple example of how the program works, just launch it and browse to your \Windows\System32 (or equivalent) folder. There will be a lengthy pause while Icon Explorer works - sometimes a minute or more - but be patient, the program is accessing every file in the folder to detail the icons it contains, and this understandably takes a while.
Once it's finished, though, you'll see a list of every file with icons, the number of embedded icons and their overall size in kilobytes. And simply clicking on a particular file will reveal a simple thumbnail display of all the icons it contains. If you'd like a closer look, just clicking on a particular thumbnail shows you all the resolutions it contains in the preview pane, and right-clicking one of these provides the option to export it in ICO or BMP format.
One problem which quickly becomes apparent is there's no way to filter the icons by size. If you're only interested in high-res 256x256 icons, say, there's no indication of where they are; you must first select individual files, then click icons one at a time to view their preview (a tedious business when some Windows files, for instance, may contain more than 100 embedded icons).
The ability to save icons in bitmaps formats other than BMP would be useful, too, although this is less of an issue as it's easy to convert them later.
Still, otherwise Icon Explorer is a clear and well-designed program which helps you locate icons much more quickly than you might do otherwise. It's also small (717KB), portable and free, so if you ever need to work with icons on your PC then it could be very useful.