Portable VirtualBox looks interesting, but it also looks like it requires the user to have admin privileges on the Windows machine that's hosting vbox. See the documentation here:
http://www.vbox.me/?path=./Description&file=ReadMe.txt
"VirtualBox needs at least main user rights, there 4 Services
(VBoxDRV, VBoxUSBMon and if not already installs VBoxUSB,
the VBoxNetFLT and sun_VboxNetFLT) to be furnished and
VirtualBox must in " Ring-3" - Mode is initiated. The drivers
the network become with snetcfg.exe (from the ms DDK 2003)
merged. So that they are loaded, must into that Attitudes of
Portable-VirtualBox, under the rider " NET" , this to be
selected. For security, which one installs, must for the
installation be agreed. After terminating Portable-VirtualBox
the drivers become and files again removes!"