In case you have a really-really bad day and your Windows 7 or 8 will not start no matter what, the bootable System Repair Disc or Recovery Drive will come in handy for restoring a system image.
If things are even worse and you cannot find your System Repair Disc/Recovery Drive for Windows, use Windows 7 or 8 DVD instead. If you can't find your Windows DVD, you can borrow one from your friends - any version of Windows DVD has Repair Your Computer option. Just make sure you use Windows 7 DVD for restoring Windows 7 and Windows 8 DVD for restoring Windows 8.
You can also download a copy of Windows 7 Enterprise 90-day trial DVD from Microsoft TechNet or Windows 8 Enterprise 90-day trial DVD from MSDN (both require logging in with Windows Live ID) using your friend's computer and burn the ISO image to DVD with Windows 7/8 Disc Image Burner or CDBurnerXP.
First, connect your external hard drive where your Windows backups are, insert the System Repair disc or Windows DVD. If your computer does not start from the CD or DVD, read the Computer boot order article to make necessary changes.
Those who use network drives (NAS, for example) for backups, must use wired connections for restoring system image. Wireless networking is not supported.
After your computer starts from the CD or DVD, a black screen with text Press any key to boot from CD or DVD appears. Press Enter or Space key on your keyboard within 5 seconds, otherwise your broken Windows will start again.
Windows will then load files from the disc. This will take some time.
For Windows 7 System Repair disc users, System Recovery Options window opens. Select your preferred keyboard layout from Select a keyboard input method box. Then click Next.
In case you booted from Windows 8 Recovery Drive or System Repair Disc, Choose your keyboard layout screen appears. Click your keyboard layout, or if one is not visible, click the See more keyboard layouts link until you see the correct layout.
For Windows DVD users, Install Windows dialog appears. Select your preferred settings from Time and currency format and Keyboard or input method boxes. I suggest you leave Language to "English" here to better understand this article. Click Next to continue.
Now you will see a big tempting Install now button. Do not click it!
Click Repair your computer in the lower left corner instead.
With Windows 7, installation(s) will then be located on all hard drives. This might take a minute or so.
As you need to restore a System Image, click Restore your computer using a system image that you created earlier. Then click Next. Make sure your external hard drive with backups is connected first!
In Windows 8, Choose an option screen appears instead. Click Troubleshoot.
Click Advanced options in Troubleshoot screen.
In Advanced options screen, click System Image Recovery.
Now click Windows 8 in the System Image Recovery screen.
Re-image your computer window opens. If you want to restore the latest available system image (the date and time of the image are displayed in Date and time: field below), leave the default selection in tact and click Next.
If you want to select an older system image instead, click Select a system image and then click Next. In the following window click the external hard drive you use for backups and click Next. If the drive is not displayed, disconnect and reconnect it and then click Refresh button.
Now select the older system image you want to restore from the list. Please note that the backup time might be different, because all times are displayed for GMT-8:00 time zone. Click Next.
Normally, you do not need to select the Format and repartition disks option from Choose additional restore options section. Just click Next.
You might need this one only if you had a total hard drive failure and you are restoring Windows 7 to a new hard disk or you had to format the present one. The term "new hard disk" also includes a used hard disk bought from auctions or friends. Just make sure the new disk's capacity in gigabytes is at least as big as the previous one!
A summary screen will be displayed. Click Finish to start the re-imaging process.
And here is the final warning that you will lose any data currently present on the hard drive selected for re-imaging. Click Yes.
Restoring a system image takes a long time - an hour to several hours, depending on the size of data on your hard disk(s). You can safely do something healthy meanwhile... 
After the system image has been restored, Windows offers to restart your computer. It will not wait for your answer forever, just 30 seconds. If you happen to be somewhere near your mouse, click Restart now. Hold your breath... not really!
Your Windows 7 or 8 should now start and work correctly. If needed, you can now restore individual files and folders from a later backup than your selected system image.
After a system image restore, Action Center icon in Windows 7 Taskbar Notification area will show a message "Solve PC issues: 1 message".
Click Action Center icon and then click Check backup settings.
Windows 7 reminds you that your backup settings might be out of date because your computer was restored to an earlier point in time. Click Keep current settings to disable the Action Center message. But please double-check that an automatic daily backup has been set up after this!
In Windows 8, you can double-check your settings by opening Settings search with keyboard shortcut Windows Key+W, typing "windows 7" and clicking Windows 7 File Recovery.
