Problem solved; I restored the Windows 7 VM from a backup, and then followed the procedure outlined here:
FUsion 6 and SATA Equals Blue Screen
... thus, after shutting down the VM, beginning with changing only the virtual CD/DVD from IDE to SATA, which installs a SATA controller and drivers in Windows 7; and only subsequently, after shutting down the VM again, changing also the virtual HD from SCSI to SATA: after booting, Windows 7 installs the virtual SATA HD device, and now everything works (after a reboot and a Windows reactivation, of course)... 
