After much poking around on the web (especially here) I eventually tried upgrading vbox.
My apt-get system insisted that I had the latest version of vbox but the Oracle/Virtualbox website offered a newer one.
It insisted that I first remove my existing virtualbox (still regarded by apt as the latest version) and guest extension software, which I did.
It then permitted me to install a slightly newer version of virtualbox: virtualbox-5.2_5.2.2-119230~Ubuntu~zesty_amd64.deb and Oracle_VM_VirtualBox_Extension_Pack-5.2.2-119230.vbox-extpack.
Now, when I try to bring up either of my windows virtual disks, I get a rather opaque error message:
I tried creating a new virtual disk for windows10, using my old virtual disk file, and that didn't work.The virtual machine 'Win10' has terminated unexpectedly during startup with exit code 1 (0x1).
Result Code: NS_ERROR_FAILURE (0x80004005)
Component: MachineWrap
Interface: IMachine {85cd948e-a71f-4289-281e-0ca7ad48cd89}
My Windows7 virtual disk doesn't work either.
I have no idea what to do now.
My bill paying software lives in my Windows10 virtual disk and I'll soon need to use it.
Any help would be greatly appreciated!
Thanks
pez