Hard Reset HTC Desire Z

First method:

1. Begin with clicking Home key to go back on Home screen.
2. Tap Menu key, and then go to Settings.
3. Next step is Storage and then Factory data reset.
4. Tap Reset phone, and tag Erase everything.

Second method:

1. Remove and reinsert battery back after waiting 5-10 seconds.
2. Press and hold the Volume down + Power button.
3. Wait couple seconds untill the three Android images appear
4. Then release the Power and Volume down buttons.
5. Use Volume keys to select Factory reset, and then press the Power button. 

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.