How to Do Hard Reset For Nokia Lumia And other Windows Phone

This procedure briefly explains how to take the Nokia Lumia 520 to factory settings but I think it works for other Windows phones as well. Follow these steps;

1. Turn off the phone
2. Press and hold the volume down and camera keys.
3. Press and hold the power key.
4. Once the phone vibrates, release the power key, but keep pressing the volume down and camera keys, until the phone reboots. All data will be lost.

Its slightly different for windows 8 phones. But these should work;
- Turn the power off
- Press and hold the volume down and connect the charger or USB cable with PC for about 8 secs, an exclamation mark (!) Will pop up on the screen. Then quickly press the following keys in these exact order;
-Volume up
  - Volume down
    - Power
       - Volume down
You could refer to Microsoft help link for more.