If you have Object Desktop, you can now try out IconPackager 2.49. IconPackager is the world's leading program for changing all your Windows icons at once by applying "packages" of icons.
Version 2.49 has a ton of very useful features and fixes including:
1) Full integration with IconDeveloper (another part of Object Desktop). So now you can instantly jump to editing and tweaking icons directly.
2) Enhanced Previews before applying
3) More icons on Windows XP supported.
4) Smart Packaging, redundant icons (duplicates) removed so that packages are smaller.
5) .iptheme encryption. The actual IconPackager theme is now encrypted (not the icons, just the .iptheme file).
If you don't have Object Desktop you can get it at www.objectdesktop.com
Update: To get you an idea of how good the smart packaging feature is - on the icon package "Toon-XP" it went from 16 megs of disk space down to 11 megs of disk space. That's 5 megabytes for the same icon package! That will save download times (and bandwidth) for everyone.