I would like to be able to use favicons that are posted here to replace the generic "empty page" favicon that comes in the bookmark(s) of pages that don't have their own. I can copy the 16x16 image to the clipboard easily -- now, how do I replace the generic favicon with the one that's on the clipboard?
It does not work with clipboard, you need to have the actual .ico file saved on your computer.
If you want to set the default favicon for your (that means you are the author) web pages, the instructions are here.
If you just want to customize the icons in your favorites pane or menu, then right-click on the item in the favorites pane in Internet Exporer and then click the "Properties" command. Finally click "Change icon" button on the property page and navigate to the saved icon.
I am not sure whether it is possible to customize favicon if you are using Firefox.
Find out how Vista icons differ from XP icons.
See how RealWorld Icon Editor handles Vista icons.