Last night I successfully flashed my HTC Desire HD to an unbranded 1.32.* ROM. The process was incredibly easy and as such thought I would share it with you. As always, this guide come with no warranty, if you brick your phone you have been warned!
The first step is to *ROOT* your HTC Desire HD, as outlined in my guide here. Once you have completed this step then read on.
The next step is to source your unbranded ROM. This link has list of a few unbranded HTC Desire HD ROMS. I chose to use the 1.32.405.3 Shipped ROM to flash my phone with. Pick a ROM (I suggest the same one I used, 1.32.405.3 Shipped) and download it to your computer. You may have noticed that the ROM has a .exe extension. That is because this ROM will be executed on the PC and transferred (flashed) to your phone via USB.
Ensure your phone has USB Debugging enabled in Settings. Connect your phone to your PC via the USB cable and execute the downloaded ROM. Follow the on screen and phone prompts to complete the flashing process.
Once completed you will have an unbranded HTC Desire HD at your disposal. If you used the same ROM that I did then you can now repeat the *ROOT*ing procedure to gain ROOT access back to your phone. Also, at this stage your phone will have none of the apps you installed. If you use Google backup then your phone, once set up with your google account, will try and restore all your previously installed apps. I found this process painfully slow and decided to cancel all downloads and manually restore the apps I wanted.