Yep, solaris runs inside Virtualbox as virtual machine on Windows host.
Prebuilt machine is just a already installed Solaris for download as a vbox image (you just start it after download), with disks and other stuff preconfigured i suppose.
I have two solaris 11 virtual machines inside Virtuabox, with solaris cluster and zone as package with my linux media center providing ISCSI luns (gave 2 cpu, and 2 GB ram to each virtual machine).
targetcli ISCSI linux implemetation support scsi3 fencing, so you can even build 3 (or more) node clusters inside Virtualbox.
Works great for testing and stuff, and to keep up between jobs