1) First you need the RUU.exe that you intend to install. The current ones for the HTC Desire HD can be found here.
2) Start the RUU. Don’t let it start flashing your device.
Search your C:\ drive (or where you have Windows installed) for “rom.zip”.
Once you found this file, copy it somewhere safe.
3) Rename the rom.zip to PD98IMG.zip and put it on to the root directory of your MicroSD card in your phone.
4) Shut down the phone.
5) Press the Power button + Volume down button at the same time and hold them untill you get to HBOOT
(white screen with skateboarding android, the button combination is different on different phones).
6) Wait for the HBOOT to parse the zip file.
7) Follow the on-screen instructions.
Boot into newly flashed device