Some of my recent exploration of virtual machines in VirtualBox led me to using a beta version of version 4. After an unsuccessful removal of a virtual machine, VirtualBox continually crashed (error relates to qtcorevbox4.dll) as soon as the interface appeared, and reinstalling the program did not resolve the issue. In order to continue using the program, one solution was to delete the virtual machine (VM) in question.
Under Windows 7, check C:\Users\USERNAME\.VirtualBox\VirtualBox.xml for the “defaultMachineFolder: property, under the “SystemProperties” node. This will show you the location of the VMs (typically: C:\Users\USERNAME\VirtualBox VMs). Deleting the VM (folder name matches VM name) that was not properly removed will fix the problem.
It has also been suggested that restoring a working copy of VirtualBox.xml might resolve such problems, but that requires an undamaged copy of the file (which if your VMs are intended to only be used a few times, might not be that practical).