I'm basically just copying and pasting out of my old post here. Some of the steps I am linking you sites and some of the steps I'm just putting my own info there because it's just easy enough to follow the few steps that I write instead of reading someone else's guide. Good luck and you should be fine as long as you follow the steps carefully (specifically when flashing the radio).
I'm not sure if you're on Windows XP or Vista/7 but if you get the rspl.cpp error when attempting to flash something, it means that ActiveSync 4.5 (XP)/Windows Mobile Device Center (Vista/7) is making USB connections and you just need to uncheck the box for that in the settings. See the thread I linked you to above for more info.
1. Flash HSPL 2.08:
How to Install HSPL3 on HTC HD2? (Video)
2. Flash radio version 188.8.131.52:
How to Install Custom Radio ROM on HTC HD2? (Video)
3. Install MAGLDR 1.13:
a. Download MAGLDR 1.13 image below and then extract it with WinRar or 7-Zip:
Download MAGLDR 1.13 Download MAGLDR 1.13 ? AddictiveTips
b. Power down your phone, then put it in the bootloader by holding volume down as you power it on.
c. Connect the phone to your PC via the USB cable (text on bottom should change from Serial to USB).
d. Run the RomUpdate.exe program and click the checkboxes. It should handle everything else for you and flash within a few seconds or so. Just follow the on screen instructions.
4. Flash the prj Clean Desire ROM:
a. Download the above file then extract its contents using either WinRar or 7-zip (both freely available programs)
b. Start ROMInstaller.exe and follow instructions.
c. Boot your device into MAGLDR by holding down the power button and select in the menu "4. USB Flasher"
d. The program will take over from here, just let it do its thing and when your phone reboots it should load up the new Android ROM.
5. Put in your APN settings. The following seem to work for US HD2 phones on T-Mobile:
The rests are blank (not set) until you get to MMSC.
MMS proxy & port are <not set>
APN type: <not set>