Now that I think it's stable enough I'm setting up 10 Pro 64 on my Wife's PC -- it's been installed for quite a while, but just not set up with her games, game clients from Amazon & Big Fish etc., and of course Steam. Step 1 as usual is monitoring the installation in a win7 32 bit VM. I've included the results below in the hope that it might be useful...
The installation performed the following activity:
3918 files added
52 files deleted
20 files updated
150 registry entries added
2 registry entries deleted
13 registry entries updated
Installed 8/11/2016 6:59:31 PM
Files/folders added...
C:\ Program Files\ Steam\
511MB - 3490 files - 398 folders
C:\ Program Files\ Common Files\ Steam
2.76MB - 2 files
C:\ Users\ [UserName]\ AppData\ Local\ Steam
128KB - 1 file - 1 folder
Registry keys added [the ones that matter, absent temporary stuff like UserAssist]...
[HKEY_CLASSES_ROOT\steam]
[HKEY_CURRENT_USER\Software\Classes\steam]
[HKEY_CURRENT_USER\Software\Valve\Steam]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run]
"Steam"="\"C:\\Program Files\\Steam\\steam.exe\" -silent"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\steam]
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Steam]
[HKEY_LOCAL_MACHINE\SOFTWARE\Valve]
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\Steam Client Service]
Notes...
Steam Client Service default = manual start.
As you can see in the above listed registry additions, Steam is set to start with Windows by default.
-------------
Installation monitoring was done using an up-to-date but minimal installation of Windows 7 32 bit in a current version VBox VM, using InstallWatch Pro. InstallWatch Pro will not work in 64 bit Windows, & it won't work in 10. I stopped the monitoring at the Steam sign-in screen, closing the Steam software, then performing the 2nd snapshot for comparison.