How does IconTweaker work, from a technical point of view? Is there a function in the Windows API that allows for changing icons without modifying the default icon files, or something else?

If you take a look at the IconTweaker installation directory, you should see a folder called Data. This folder contains 8 XML files (cursors.xml, desktop.xml, drives.xml, files.xml, folders.xml, network.xml, other.xml and start.xml). Each folder contains data for your current icons. Also, as you can see when you start IconTweaker, there are 8 tabs. So when you change an icon in IconTweaker it changes your icon directory in one of your XML files too. And I think Windows then knows where too find the icons.

(I guess all original Windows icons are still somewhere on your computer... maybe the WINDOWS directory).

Btw each XML file in the Data directory looks like this:

CODE<ClosedFolder>
<itd:DisplayName>Closed Folder</itd:DisplayName>
<itd:DefaultIcon>
<itd:All>%System%\shell32.dll,3</itd:All>
</itd:DefaultIcon>
<itd:RegKey>
<itd:All>3</itd:All>
</itd:RegKey>
<itd:RegType>
<itd:All>ShellIcon</itd:All>
</itd:RegType>
</ClosedFolder>

Most (but not all, which is often very annoying) default Windows icons are inside of shell32.dll, in Windows\System32. IconTweaker doesn't modify this file, but it must be modifying some Windows system file because modifications to icons persist after uninstallation and because even if it was storing icon information itself, it would have to tell Windows were to look for the information somehow.

I think Windows just 'knows' some icons are also stored in the IconTweaker/Data directory too, so it doesn't load them via shell32.dll. I hope you understand this?

To clarify my last post: I've uninstalled IconTweaker, which leads me to believe that Windows stores icon information somewhere other than the IconTweaker data folder (which no longer exists).

The same effect can be achieved by enabling the generic "Internet" link on your XP Start Menu, and then installing Firefox: the Internet Explorer icon is replaced by a Firefox icon (the default "Internet" icon is stored in iexplorer.exe, which is not altered when Firefox is installed).

The generic Internet link changes according to the default browser. When you set FF as the default browser, it probably changes a registry key, which then causes the generic link to change..

But of IconTweaker, I am not sure..

I`m not sure if you mean this ?

Windows stores icons in a database called "Iconcache.db". It`s possible that you customize some icons but they don`t change, thats why windows use the "Iconcache" and don´t read in all new icons.

You can refresh the icon cache by deleting the IconCache.db file from your profile directory (usually CDocuments and Settings/Username/Local Settings/Application Data - it`s a hidden file/folder). Windows will create a new file.

Microsofts TweakUI do the same. Download TweakUI (from Microsoft, right side), install it and at the bottom you have the option "REPAIR", chose repair icons (reboot) ...

yeh i agree, tweakui will do it all

Yeah TweakUI does work cool!!

I don't think both of your comments add any value of discussion to this thread

To clarify: the new icons remain altered after I uninstall IconTweaker. I've manually removed both IconCache.db and the local thumbs.db several times in the course of modifying shell32.dll (and explorer.exe, which for some inexplicable reason contains the "My Computer" icon) to no effect.

There are 2 reasons when you change/edit systemfiles but they don`t change the icons, first is the iconcache.db, Windows always use the Iconcache (often changes at the system will not displayed because Windows use an old iconcache). Second, your customized systemfile will be overwritten by the system (Prefetch-folder, DLL-Cache, System File Protection).

When you change some icons/dlls/exe - refresh your iconcache

I'm perfectly aware of all of that: I'm not having trouble changing icons, I'm having trouble figuring out how IconTweaker changes icons without altering the underlying system files.

Here is an example.. [I am not sure that IconTweaker works like this - just a theory]

The my computer icon is determined by a registry key:
HKEY_CLASSES_ROOT\CLSID\{20D04FE0-3AEA-1069-A2D8-08002B30309D}\DefaultIcon
On that key, the "Default" string has a value of "icon_file_name,icon_number" - for example: explorer.exe,0

Now, when IconTweaker changes those values, the my computer icon changes.
Again, I am not sure if IconTweaker works this way..

Each system icon has a registry value. By changing that value, the icon changes. For example, my HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Current Version\Explorer\Shell Icons looks like this:

ssdad.PNG

So, I guess it works so, that if you want to assign a custom icon to a specific system icon, you need to add a registry value (if the specific registry value for a specific icon does not work, it uses the default icon..

As you can see from the image above, the strings in Explorer\Shell Icons has a number (e.g. 29). From the list below, you can see which number you need to add to assign a specific icon to something.. E.g. if you add number 8 and the string to  explorer.exe,0, the hard disk icon will change to the My Computer icon.

CODE  0 (Explorer) Unknown File Type
  1 (Explorer) Default Document
  2 (Explorer) Default Application
  3 (Explorer) Closed Folder
  4 (Explorer) Open Folder
  5 (Drive) 5.25 inch floppy
  6 (Drive) 3.5 inch floppy
  7 (Drive) Removable Drive
  8 (Drive) Hard Drive
  9 (Drive) Network Drive
10 (Drive) Network Drive disconnected
11 (Drive) Cd-ROM Drive
12 (Drive) RAM Drive
13 Entire network (globe)
14 ??
15 (Explorer) Networked Computer
16 (Explorer) Printer(s)
17 (Desktop) Network Neighborhood
18 (Explorer) Workgroup
19 (Startmenu) Programs
20 (Startmenu) Recent documents
21 (Startmenu) Settings     22 (Startmenu) Find
23 (Startmenu) Help
24 (Startmenu) Run
25 (Startmenu) Suspend
26 (Startmenu) Docking
27 (Startmenu) Shutdown
28 (Overlay) Sharing
29 (Overlay) Shortcut
30 (Overlay) ??
31 (Desktop) Recycle bin empty
32 (Desktop) Recycle bin full
33 (Explorer) Dial-up Networking
34 (Explorer) Desktop
35 (Startmenu) Settings/Control Panel
36 (Startmenu) Programs/Program folder
37 (Startmenu) Settings/Printers
38 ??
39 (Startmenu) Settings/Taskbar
40 (Explorer) Audio CD
42 (Explorer) Saved search (.fnd)
43 (Explorer & Startmenu) Favorites
44 (Startmenu) Log Off

SOURCE: http://www.virtualplastic.net/html/icn_reg.html

I hope this clears things up

Interesting. I've done some experimenting with the Shell Icons keys before, and they seem to have very limited control over icon specification (and there appear to be differences between versions of XP, and possibly between installations of the same version), but I was unaware of the HKEY_CLASSES_ROOT keys. (Is there any documentation of the registry system in its entirety, official or not?) Thanks for the information, I'll have to do some more experimenting.

http://www.virtualplastic.net/html/icn_reg.html
That site contains a lot, and I'm sure you can find more with Google