I mainly run Windows VMs on a Linux host. When you update to the latest version of VirtualBox, it’s best to update (or install) the guest additions.
Normally, you can run the installer by selecting Devices->Install Guest Additions from the VirtualBox menu. This doesn’t always seem to work though, so it’s necessary to run the installer manually.
On a Linux host, the guest additions can be found in /usr/share/virtualbox/VBoxGuestAdditions.iso. If you share this folder with the guest, you can mount the ISO directly using various free tools. However I find it easier to just copy it to the guest, use 7-zip to extract everything, and run VBoxWindowsAdditions.exe.
Look on my works, ye Mighty, and despair! 