OK, I just solved this one myself after much, much pain.
In the C:\WINDOWS\SYSTEM32\DRIVERS directory, there is some driver loaded during regular boot and not safe boot.
In my case, I renamed three driver files:
ipvnmon.sys to ipvnmon.sys.renamed
sonydcam.sys to sonydcam.sys.renamed
sonyfanc.sys to sonyfanc.sys.renamed
Alternately, look for unsigned driver files using sigverif.exe
The Sony drivers of this particular guest (converted Windows XP machine) seemed to be a real problem.