There are many different ways to get icons for your application. One of them is to design those icons yourself using one of the widely available icon-editing programs. But that could be a hard task, if your drawing skill are low or not present at all. The other way is to use a search engine and get the icons free of charge from someone, who does have some drawing skill and therefore, managed to create the icons and put them on the net. There is only one problem with those icons - their quality. You can usually see rough edges, inconsistent color and blurry look in smaller sizes. That does not apply to the images, created by professional designers. However, professionally designed icons are not usually offered free of charge. Not usually, does not mean not at all. And here is an exception.
Windows NT 3
Windows NT 4