clipped from: notepad-plus.sourceforge.net   

1. Unzip all the files into a directory, say C:\foo\.
2. Open the toolbarIcon.xml which comes with this package with your favourite editor (Notepad++, of course ;) ). replace E:\Sources\notepad++\PowerEditor\bin\newIcons\ by C:\foo\ in the tag<Theme name = "Demo" pathPrefix = "E:\Sources\notepad++\PowerEditor\bin\newIcons\"> then save it.
3. Get Run Dialog by clicking start->run, type %APPDATA%\Notepad++ into the text field of Run Dialog, and then type ENTER. A folder named Notepad++ will appear. Copy the modified toolbarIcon.xml into opened folder.
4. Now it's the magic moment: Close the Notepad++ then re-launch it. You'll see the Crystal icon set instead of the default icons.