This seems to work for me since the vista x64 installer just created an empty directory.
msiexec /a ati-stream-sdk-v2.0-beta2-vista64.msi /qb TARGETDIR=c:\opencl
You can leave it there or copy it there or copy "c:\opencl\program files\ATI STREAM" someplace else. I copied it to c:\opencl and deleted the temp files.
Add the environment variable ATISTREAMSDKROOT=C:\opencl\ATI Stream.
Add "C:\opencl\ATI Stream\bin\x86" to your path.
This should be all thats needed to get it running.