If you look at either of the following keys:
You can find a list of the SIDs there with various values, including where their “home paths” which includes their usernames.
- Open Reg HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\
- make a loop to get all subkeys
- the subkeys you are interested with are those started with [S-1-5-21-] which means user (see key name [ProfileImagePath] they are always started with a path c:\Users)
- Those starting with [S-1-5-21-12] are all local users
- Those starting with [S-1-5-21-13] are all network users [if joined to Domained network] that are previously logged on the machine.