I guess it will really depend on which HD2 you have.
If you have a T-Mobile branded HD2 you can get the ROM file HERE
off my DropBox.
If you have a different one, then let me know and I will link the ROM you need but regardless, the steps are the same.
1. Download the original Stock ROM/SPL for your appropriate HD2
2. Extract the .zip file the ROM comes in (if it comes in one)
3. Plug your phone in via USB
4. Sync it with Windows Device Center 6.1 and wait for it to finish
5. Double click the .exe file inside the folder we extracted earlier
6. Follow the prompts and wait for it to finish flashing
7. Once it is done, click ok, close the program, and reboot the phone. Done!