The Windows installation CD is very helpful if you want to restore Windows 7 to default. But you can still do this without a CD, just follow this awesome article of TechLinu. But do not forget to back up your personal data and programs beforehand.
If your computer’s manufacturer has stored the Windows 7 installation on your PC, carry out the following steps: open the Control Panel and go to the category “System and security”. Under “Back up and restore”, you will see the item “Restore system settings on the computer” below. Next, click on “Advanced restoration methods”, select the option “Reset computer to its factory settings”, and follow the instructions.
However, if you stumble upon the option “Re-install windows (Windows installation data storage device required)”, you must reset Windows 7 using the recovery method. When your computer starts, look out during the booting process, for the note at the lower corner of the screen that tells you to press a function key to go into the recovery mode. This is usually done with the [F11] or [F9] keys; if not, try the other function keys. On certain notebooks, the [Fn] key at the bottom left of the keyboard has to be pressed in order for the function keys to be operated. If you haven’t responded fast enough, you’ll have to repeat the booting process.
Once your computer is in recovery mode, select the item “Recovery” with the arrow key. Since you’ve already backed up your data, you can enter the next step by clicking on “Next”. Windows will now start the restore process.