I suggest installing the package from virtualbox.org, because VirtualBox in the repositiory is outdated.
Go here https://www.virtualbox.org/wiki/Downloads
then choose to download 4.2.6 for Linux, then choose the version of Ubuntu that corresponds to the version of Mint you're using. For example, Nadia: 12.10, Maya: 12.04 and so on. You should also download the Extension Pack for all platforms to add extra functionalities, such as USB handling.
Then, double-click the package you've downloaded. GDebi will pop up. Click Install Package and type your password, then wait for a moment for the installation to finish. You should see VirtualBox in the menu now
Finally, to install the Extension Pack for it, double-click the extension pack.
Done! Now you can add a virtual machine and set up Windows XP