How to Flash Windows Phone
Various ROMs are available via the XDA-Developers.com website and forum - you will have to perform a search for ROMs for your model to check whether they are available. ROMs for the HTC HD7 can be found via the site, and this will be your single best resource.
The ROM will be in ZIP format, so once downloaded, unzip into a separate folder.
Next, switch off your Windows Phone by holding the power button until the "Goodbye" message appears. With your phone switched off, press and hold the volume-down button and then hold the power button to enter the bootloader mode, which will be identified by a screen with three colored stripes. Release the volume-down button when this launches, and connect your phone to your Windows XP or later computer with the supplied USB cable.
Your phone should be identified and drivers installed by Windows; check your phone's display, where "USB" should be displayed. This confirms that the USB connection is successful.
It is now time to flash the ROM to your phone. Begin by visiting the folder where the downloaded ROM file was unzipped to, and then double click the ROMUpdateUtility.exe file.
In order to ensure success and avoid bricking your phone, you must follow the instructions on your PC/laptop screen to the letter.
Completion of this process may take a short time, which is why you should not attempt it on a battery-powered laptop, for instance.