Solved: How to reset HTC Desire HD hard reset: With the phone turned off, press and hold the VOLUME DOWN button, and then...

Restarting or factory resetting your phone

Restarting your phone (soft reset)
If your phone is running slower than normal, an application is not performing properly, or your phone becomes unresponsive, try restarting your phone and see if that helps solve the problem.
If the display is off, press the POWER button to turn it back on.
Unlock your phone if the Lock screen is displayed.
Press and hold the POWER button, and then tap Restart.
When asked to confirm, tap Restart.
If you can't restart the phone following the above steps, remove the battery, wait for a few seconds, reinsert the battery, and then turn the phone on.

Performing a factory reset (hard reset)

If you're passing your phone on to someone else or your phone has a persistent problem that cannot be solved, you can do a factory reset (also called a hard reset). This returns your phone to the state it was in before the first time you turned it on.
Factory reset will not erase the data and files that are on the storage card.
Factory reset is not a step to be taken lightly. It will permanently delete all your data and customized settings, and will remove any applications you've downloaded and installed. Be sure to back up any data and files you want to keep before you do a factory reset.

Performing a factory reset using phone buttons

If you can't turn on your phone or access the phone settings, you can still perform a factory reset by using the buttons on the phone.
With the phone turned off, press and hold the VOLUME DOWN button, and then briefly press the POWER button.
Wait for the screen with the three Android images to appear, and then release the VOLUME DOWN button.
Press VOLUME DOWN to select FACTORY RESET, and then press the POWER button.

Performing a factory reset via settings

From the Home screen, press MENU, and then tap Settings.
Tap SD & phone storage > Factory data reset.
On the Factory data reset screen, tap Reset phone, and then tap Erase everything.