Thank you for your help!
I reconfigured and recompiled the qt-win-opensource-4.7.3-vs2008.exe (after "nmake confclean") and added the -no-s60 switch. I had to restart nmake a few times (had problems updating some files and stopped), but it finally succeeded.
Conclusion: 4.7.3 works with default configuration on my Windows XP 32-Bit, Windows 7 32-Bit and Windows Vista 64-Bit machines, but needs the -no-s60 switch on my Windows 7 64-Bit machine. Strange behaviour? Who cares if it's working ;)
Andreas.