How to Flash the ZIP File; PB99IMG.zip
Please, note that this process may render your phone useless if carelessly used and that, interruption of the process for any reason or any means is destructive therefore; you should exercise caution going through the process.
Also, your battery must be at least, 30% charged before you proceed.
1- Download the file named PB99IMG.zip in the table below this page.
2- Copy the file to the root of your SD Card i.e don't copy it under any folder in the SDCard. And make sure it is named PB99IMG.zip
3- Disable fastboot mode on the phone by going to <b>Menu <<>> Settings <<>> Power <<>> Fastboot.
4- Put off your phone or remove the battery and re-insert the battery again.
5- Boot the phone to hboot by pressing the Power + the Volume Down buttons together for some seconds until the phone boots to hboot.
6- Wait few seconds for the phone to detect and scan the PB99IMG.zip file on your SDCard. If the phone prompts that; "Do you want to start the update?" press the volume Up button to start the update.
7- Wait few seconds for the update to complete and reboot your HTC Desire.
Congratulations! Now, you can install any custom ROM, upgrade or downgrade your HTC Desire and lots more!
Visit the Source Link below to download the PB99IMG.zip file.
Source: http://www.arykblog.com/2014/02/pb99imgzip-download-link-htc-desire.html