Windows PCs have many .dll resource files that contain number of icons. Here are some of them.
C:\Windows\System32\shell32.dll
C:\Windows\System32\compstui.dll
C:\Windows\System32\DDORes.dll
C:\Windows\System32\ieframe.dll
C:\Windows\System32\imageres.dll
C:\Windows\System32\mmcndmgr.dll
C:\Windows\System32\netshell.dll
C:\Windows\System32\wmploc.dll
C:\Windows\System32\pifmgr.dll
This is taken from a Windows 7 PC, but most of these .dll files will be available in other Windows versions as well.
Great collection!
Like C:\Windows\System32\pifmgr.dll, here's one more from the Windows 3.1 days that contains icons for many of the apps from back then, as well as a few generic MS-DOS icons: C:\Windows\System32\moricons.dll