I have now tried various combinations of BIOS switches (enable/disable USB, assign IRQ to USB or not, enable/disable ACPI) and boot options (noapic, acpi=off, pci=noacpi), and nothing works - it looks like my USB controller is not going to work with my Linux version.

Oh well... I have gotten myself a CompactFlash-to-IDE adapter now, which means rebooting my system every time I want to change the cf-card, but which works otherwise well, so I can do without USB. Thanks to everybody who hast tried to help !

Heiko