If you dual boot between Windows NT 4.0 and Windows 2000, attempting to load the Windows 2000 System hive, while booted to Windows NT 4.0, generates a BSOD:

STOP: 0x0000001E (0xC0000005, 0x801A8A83, 0x00000000, 0x00000024)                              KMODE_EXCEPTION_NOT_HANDLED                              Address 801A8A83 has base at 80100000 - Ntoskrnl.exe
Unless you also have an alternate Windows 2000 install on this computer, the only way I know to load an inactive Windows 2000 System hive is to copy it to another Windows 2000 computer. Then you can use Regedt32 to Load Hive. If you modify the hive, copy it back to the inactive instance of Windows 2000.