Hi compo,
Rooting the handset would invalidate the warranty, so I would not recommend this.
As you've tried multiple downloads, this rules out the possibility that the downloaded file is corrupted, so I'd start by backing up the handset to protect all your date. Once you've done that I'd reset the Desire HD to its factory settings.
Once you've done this, that's when to try a clean install of the update, before you restore any data or reinstall any apps so that nothing from these can affect the update. Once completed, you can then reinstall everything on the new firmware.
If the install still fails for any reason, then I'd suggest sending the handset for repair so that we can try for you and investigate the issue if anything should come up.
Dave
