Completely reset my Desire HD
That's one of the most easy tasks, and is called Factory Reset. Depending on your Android version, you find the suitable item at different places in the device's Settings menu:


Android 4.0 has it in Backup and Reset (at the end of that screen, watch out for "personal data")
Android 2.3 placed it in Privacy (with a fence-symbol; again at the end of that page)


There are also ways to achieve this without going via the settings menu, booting into Recovery Mode. hard-reset.com mentions the appropriate procedures for most devices.