If you want install XAP files without a developer account you can do it by modifying some registry keys:

1. First go to MFG and follow the path 7.Engineer Menu > 6.Other Settings > Edit Registry
2. Set the following fields:
   • Select ROOT_PATH: HKEY_LOCAL_MACHINE
   • Input SUB_PATH: Comm\Security\LVMod
   • Input KEY and Select data type:  DeveloperUnlockState   DWORD
   • Input data: 1
3. Click Set button

To avoid Zune undo to default settings:

1. Go to MFG and follow the path 7.Engineer Menu > 6.Other Settings > Edit Registry
2. Set the following fields:
   • Select ROOT_PATH: HKEY_LOCAL_MACHINE
   • Input SUB_PATH: Software\Microsoft\DeviceReg
   • Input KEY and Select data type: PortalUrlProd   DWORD
   • Input data: leave this field empty
3. Click Set button

1. Go to MFG and follow the path 7.Engineer Menu > 6.Other Settings > Edit Registry
2. Set the following fields:
   • Select ROOT_PATH: HKEY_LOCAL_MACHINE
   • Input SUB_PATH: Software\Microsoft\DeviceReg
   • Input KEY and Select data type: PortalUrlInt   DWORD
   • Input data: leave this field empty
3. Click Set button

From now on your device is unlocked in developer mode, but you cannot install homebrew apps that modify registry keys. You need Interop Unlock:

1. Go to MFG and follow the path 7.Engineer Menu > 6.Other Settings > Edit Registry
2. Set the following fields:
   • Select ROOT_PATH: HKEY_LOCAL_MACHINE
   • Input SUB_PATH: Software\Microsoft\DeviceReg\Install
   • Input KEY and Select data type: MaxUnsignedApp   DWORD
   • Input data: 300
3. Click Set button

NOTE1: if you want install very nasty apps then you need WP7 Root Tools.

NOTE2: Unlock & Interop Unlock works on WP7 Tango too.

NOTE3: to lock device again set DeveloperUnlockState to 0 (zero).