virtualbox.org

I'm running VirtualBox 4.1.10 r76836 on Windows 7 64-bit SP1 with 16GB RAM. I've got a virtual machine that was working fine a few weeks ago and is now not. It's running CentOS 64-bit (2.6.32-220.23.1.el6.x86_64; identified to VBox at "Linux/Red Hat 64-bit") with 2GB RAM. As of now, the GRUB menu works, but when I select the OS I get "Error 13: Invalid or unsupported executable format. Press any key to continue..." I can return to GRUB, but not boot properly.

I may have updated VBox in response to an update prompt at some point, I don't recall. Other than that, nothing should have changed between now and the last time I ran it. I've always been careful to shut it down properly. I've tried googling for the error, but none of the results seem to be directly applicable. Any thoughts?

This usually means a grub configuration error, or corrupt MBR. Since you actually get to grub but fails to boot. Have you tried to see if you could rebuild grub from recovery?
I really don't see how this would be an issue with VirtualBox. I would be looking at some outside cause if you are certain that you shut down the guest clean. Some outside causes could be HDD imminent failure, host defrag on the VDI, maybe host virus software.