Thank you all for your interest. To clarify: I'm using a Windows system and the (fairly new) processor (of course) supports SSE as well as SSE2, SSE3 and others.
With the debugger (WinDbg) I tracked the problem down to the OpenCL installation on my system. I was using an older version of "AMD APP SDK Runtime" which provides OpenCL.
Once I removed this package, Partha's GIMP started working.
So "Odinbc" was right, it had to do with the graphic drivers in the wider sense.
Case closed.
P.S.: The most valuable hint came from
http://mrlithium.blogspot.de/2013/03/opencl-hassles-ati-opencl-driver-bug.html, people in the field of mining bitcoins also have problems with OpenCL in their applications.
P.S.2: Partha himself suggested to set the environment variable GEGL_ENABLE_OPENCL to no, but I didn't try that.
P.S.3: Using GPU-Z would have detected the OpenCL problem immediately. With the faulty OpenCL package installed, it displays an error message.