I am getting the same result trying to do a new install of Linux Mint x64 v8.0 - this was working recently (as an installed vm) and stopped working today. I run Win 7 Enterprise x64 as host. I have had a problem with defective memory recently, which I replaced. I installed the new version of VirtualBox a few days ago. I uninstalled virtualbox and reinstalled with a copy freshly downloaded.
Since the existing vm didn't work I tried creating a new one and the reinstall complains of the wrong arch (it thinks the vm processor is 32 bit) and I am trying to install 64 bit.
For my vm I can always install 32 bit, but not certain why I am getting the arch complaint now.