Hard Reset HTC Desire S

First method:

1. From the Home screen, press Menu button , and then tap Settings.
2. Tap SD & phone storage then Factory data reset.
3. Tap Reset phone, and then tap Erase everything.

Second method:

1. Remove the battery, wait for a few seconds, then reinsert the battery.
2. Press and hold the Volume down button, and then briefly press the Power button.
3. Wait for the screen with the three Android images to appear, and then release the Volume down button.
4. Press Volume down 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.