If Windows does not start at all or reboots or stops responding while starting, the first thing to try is Last Known Good Configuration. This one is especially useful when the problem appears right after some software or driver installation or upgrade, or when you edit registry manually or change advanced system settings.
With some luck, this troubleshooting option can even get around the dreaded "C:\windows\system32\config\system is missing or corrupt" errors. If Windows starts properly after this, you must verify that there are no events with ID 7 concerning your PC's hard drive in Event Viewer's System log. If there are, the drive is about to fail and you must replace it as soon as possible.
This option is available in Windows XP, Vista and 7; in Windows 8, try using Safe Mode instead.
Windows always monitors if it starts and works correctly. If everything is fine, Windows saves its good configuration to a special hidden folder (I will not tell you its name and location, you curious minds! ).
To get access to restoring the Last Known Good Configuration, you will have to press F8 key on your keyboard after the big logo or black screen with white texts appears.
On some computers you will see a boot selection after pressing F8, cancel that by pressing Esc key on your keyboard and then press F8 key again.
First you might see an operating system selection screen. Select the correct one with arrow keys and press Enter key. If you already pressed F8 repeatedly, this screen might not appear.
You will see screen like follows. Use ARROW DOWN key on your keyboard to move to Last Known Good Configuration (your most recent settings that worked) (in Windows XP) or Last Known Good Configuration (advanced). Press ENTER key to start Windows with Last Known Good Configuration.
If Windows starts and works normally now, you do not have to do anything more, just keep working with it.