I have been using Virtualbox for about a year without problems. But a few weeks ago it started giving an error when ever I try to start a guest OS. It says:
"VT-x/AMD-V hardware acceleration has been enabled, but is not operational. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot."

I am running the amd64 version of debian and the guest is the win XP 64 bit.

I have tried googling the error and found a fix. By doing a cold shutdown i.e. turning the computer off unplugging it and letting the capacitors drain, then starting it up again. That fix only works for people that have suspended their computers, since I guess it doesn't set things correctly after it starts up again. But I tried it anyways and it didn't work. I have also tried turning the hardware accelerated virtualization off just to see what would happen and the error doesn't change. It still says it is enable but not operational. I have also reinstalling Virtualbox but that didn't help either. Does anyone have any ideas on how to fix this?

Are you sure the CPU on your host supports VT-x/AMD-V?

Adam

One assumes so given this ...You didn't do an update of VBox by any chance ?. I've had situations (Windoze host) where I had to regress - and it doesn't do a very good job of uninstalling itself. I don't use it on Linux hosts, so may not be relevant to you.

Sorry, I missed the part about you having used VirtualBox for about a year... Not sure what's going on, then.

Yes I did update it. But don't remember if that was when it stopped working or not, it was a while ago and I don't use it very often. When I was trying to get it working I did try to use on of the older releases but I couldn't get it to run. There was some problem with the kernel module. I think I know what the problem is now so I'll try an older release and see if that works.

I downgraded and it's working again. I had to reinstall windows since the old install wouldn't work but at least it's running now.