How to change the proxy setting for PC system account if you don't know the password of PC system account?
1 Login the PC machine by using any account
2 open registry HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\
3 make a loop to get all subkeys, check [ProfileImagePath] and find the name of PC system account in it (key name is always started with a path c:\Users)
4 Go to HKEY_USERS\user ID\Software\Microsoft\Windows\CurrentVersion\Internet Settings. (user ID in the subkey that contains the PC system account).
5 Change the value of ProxyEnable (0=disable, 1=enable) and ProxyServer(proxy:port)