Hi All,
I haven't gone through all 32 pages of this thread, but I had similar problems:
Windows 10 did the major upgrade and VBox wouldn't start at all, and it wouldn't even get far enough to leave me any error logs. Luckily, by browsing through this thread and another thread (the one on hardening issues) this is what fixed it for me:
PC: Acer aspire v10 nitro black edition, Intel i7-4710HQ @ 2.5GHz, 16GB RAM, 500GB SSD, 1TB HDD
OS: Windows Home 10, version 1511, build 10586.0 (upgraded from win 8.1 to win 10, then just got the big win 10 update on Sunday 6th Dec 2015)
VBox: Originally version 4.xx, working fine with vagrant, win 10 host and ubuntu64 VMs (data-science box from vagrant atlas, but reconfigured to run Xubuntu in Gui mode)
Fix:
From this link: (won't let me post the link because I have a new account

but it was a stackoverflow post with the following title: "failed-to-open-create-the-internal-network-vagrant-on-windows10"
Specifically this command worked when I downloaded VirtualBox-5.0.11-104061-Win.exe
C:\Users\Me\Downloads>VirtualBox-5.0.11-104061-Win.exe -msiparams NETWORKTYPE=NDIS5
No reboot needed, found all my VMs and everything came up perfectly.
I hope this helps someone...