Another debugger for x64
maybe a batter choice than windbg

Main features
Fully support 64-bit native processes
Fully support 64-bit .NET processes
Full code analysis
Full memory analysis
Code edition
Memory edition
Module export formats (EXE/DLL/CSV)
Debug multiple processes
Debug multiple child processes
Detour exported functions (thread-safe)
Detour COM methods (thread-safe)

you can download it at:
http://www.duxcore.com/fs_files/VisualDuxDbgSetup.zip