This guide is for Windows users who don’t feel comfortable with Method #1.  It’s still really quite easy to follow as you’ll see…
Whilst these instructions will work, I’ll try to go into a bit more detail tomorrow and hopefully get a video tutorial up too.

Disclaimer: I take no responsibility for anything that may go wrong by you following these instructions.  Proceed at your own risk!

What You’ll Need

  1. unrEVOked3 for Windows – download

Preparation

  1. Uninstall HTC Sync if you have it installed
  2. Install the hboot driver using these instructions (it says Evo, but works for Desire) – when complete restart your phone
  3. Reinstall HTC Sync
  4. Double-click the reflash-bundle.exe file you just downloaded
  5. It will ask you where to install it, so just pick a location on your computer you can remember

Rooting

  1. Go to where you installed Unrevoked
  2. On your Desire go to Settings > Applications > Development and make sure you have USB Debugging turned on
  3. Connect your phone to computer via USB
  4. Run reflash
  5. Wait a couple of minutes whilst your phone is rooted
  6. If all goes well you’ll have a rooted phone

Final Word

Once rooted, you can install HTC Sync again.

At the end of this guide your phone should be rooted and you’ll be in recovery mode.  At this point you can restart your phone and continue using it normally, but with the bonus of having root permissions.
You’ll be able to run apps that need root permissions like ShootMe for screenshots, and Titanium Backup which lets you make a backup of all your apps and the app data (really suggest you do this if you’re going to install a custom ROM)

You can now move on to installing custom ROMS including the leaked version of Froyo if you like :)

UPDATES

  1. 17 July 2010 09:29: Added “Reinstall HTC Sync” step.  This will make sure the phone is always recognised and prevent it from being reported as “disconnected”.  Still need to uninstall so that you can install the hboot driver without interruptions.
  2. 21 July 2010 19:18: Added note that phone should be restarted after installing hboot drivers