VirtualBox do not start: KVM

Maybe it also happens to you: you try to start a virtual machine from VirtualBox and you hit an error message: "Disable KVM/VMX extensions" or something similar. The problem is: for some reason the new kernels (after 2.6.25) load some extensions. You just need to remove them. Execute the following as root:

Search for the modules:
#---
lsmod | grep kvm
#---

If there is any loaded module, unload them, in my case (respect the dependency order):
#---
rmmod kvm_intel
rmmod kvm
#---

Now try to start your machine. For me it did the trick.