Could you do a bit more debugging? In the guest, please open regedit.exeand add/edit the following keys:
HKLM\SYSTEM\CurrentControlSet\Services\VBoxService\ImagePath = "system32\VBoxService.exe -vvvv --logfile c:\VBoxService.log" (REG_EXPAND_SZ)
After the registry modification were made the guest needs to be restarted. Please then attach the VBoxService.log file(s) (will be written to C:\ on the guest) to this defect. Thanks!