There's one key where several system icons are stored. This is
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Current Version\explorer\Shell Icons
Here's a list (permanently under construction):
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