Where are the Windows "SDK tools"? Where is ildasm? (Lucian Wischik)

C:\Program Files\Microsoft SDKs\Windows\v6.0\bin\ildasm.exe
C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin\ildasm.exe
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\ildasm.exe
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\x64\ildasm.exe
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\ildasm.exe
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\x64\ildasm.exe
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin\ildasm.exe
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin\x64\ildasm.exe
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\ildasm.exe
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\x64\ildasm.exe

What are all these different versions for? Which one should I use? Sumit Kumar is the Project Manager for “Windows SDK”, so I sat down with him to find out. Full details on my blog, http://blogs.msdn.com/lucian/archive/2008/11/15/where-are-the-sdk-tools-where-is-ildasm.aspx

(Note: the last four directories exist but don't actually contain a copy of ildasm.exe. I just put them there for dramatic effect.)