Hard Reset HTC Desire U

First method:

1. Go to Home screen, select Menu button , and then select Settings.
2. navigate to SD & phone storage option then Factory data reset.
3. select Reset phone, and then mark Erase everything.

Second method:

1. Remove the battery from HTC device and replace it after few seconds.
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.
4. Then release the Volume down button.
5. Press Volume down and navigate to Factory reset option, 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.