|Perform a "factory reset" as explained below (source: HTC website):|
NOTE: A Factory reset will permanently delete all your data and customized settings, and will remove any applications you've downloaded and installed. SIM card data will not be deleted.
To perform a factory reset on your device:
With the device powered off, press and hold the VOLUME DOWN key.
While continuing to hold VOLUME DOWN, press the POWER key for 10 seconds. You will see the Back, Home and Recent apps keys blink while holding down the POWER key.
When the keys have finished blinking, release the POWER key. While still holding VOLUME DOWN, press and release the POWER key one more time.
When the screen with the three Androids is displayed, release the VOLUME DOWN key.
Press VOLUME DOWN to select FACTORY RESET, and then press the POWER key.