In order to hard reset a HTC Desire, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
HTC Desire device will be brought back to the default configuration of a brand new device.
1. First, hold down Power & Volume Down buttons.
2. Wait until a new menu shows on screen.
3. Select Clear Storage by using Volume Down to navigate and Power button to choose the option.
4. Unless you messed something up, phone will now reset and reboot. Good work.