WARNING: Everything will be erased out of the phone!

1. Turn the phone off

2. Press and hold down the VOLUME DOWN (red box) and ENTER (green circle), then press the POWER button on the top of the phone.

If above doesn't work, try 2nd method below.

1. Press and hold down the POWER button, VOLUME DOWN button and the ENTER button.  While holding down all 3 buttons, use the stylus and press the small reset hole on bottom of phone.

If none of the above works, then you can do this:

Go to Start Menu, then Settings, then System and select Clear Storage.