Thanks for the immediate response.
My PC is already 2 years old and may not really meet the NX4 requirements.
It's a
Siemens-Fujitsu Scaleo 600
AMD Athlon 64 3200+
1GB RAM
250 GB HD in 2 partitions
Video Radeon 9600XT

Do I need a hardware (video-) upgrade?

The problem will not be fixed by re-installation and there are no special messages during installation that may give any indication on a later occuring problem, just quite a normal installation procedure.