A recent (past week) seems to be preventing the dropdown from having 64-bit Guest listed.
I have also uninstalled the Visual Studio 2015 preview (which might have had HyperV enabled) HyperV is now disabled.
this was working, BIOS is set correctly. Dual boot into Linux and run VB from there does show 64 bit guest listed.
Win 8.1 64bit
Any suggestions?
[Resolved]
ran bcdedit /set hypervisorlaunchtype off