virtualbox.org

I've also reinstalled Avast Free AV, and VirtualBox still works.

Plucking up the courage to do the same with Malwarebytes.

I was having this same error - which appeared to start following a Windows update - however uninstalling Trusteer Rapport also did the trick for me: I reported this issue to them in their "Why did you uninstall Trusteer Rapport?" page that I was directed to after uninstalling it.