Have you tried reinstalling the driver for whatever networking device you are using if you are using one?
The laptop was connected to wireless network all the time. So, I check the Dell website (the laptop is a Dell Inspiron 13) but there is only BIOS driver available for download. I found another driver by the adapter name Dell Wireless 1397 WLAN Mini-Card, but during installation, it says no compatible hardware is found.
I would run sfc /scannow 3 or 4 times...re-booting after each.
I've run another 3 times in normal mode, and another 3 times in safe mode, restarting each time, still same result.
Sorry - should have seen that. Do you by any chance have a restore point or, preferably, disc image from a time before the problem arose? If not, try replacing the file in SAFE MODE.
System Restore was turned off before this, so no restore point, no disk image.
Replacing the file in Safe Mode still prompt for administrator permission.