Portable Ubuntu for Windows is an Ubuntu system that runs as a Windows application (no install required: just unpack/extract and run)
* Ubuntu version: 8.04
* uses CoLinux
* It is not a "traditional" Virtual Machine (like VMware Workstation or VirtualBox)
* Some (intensive) graphical operatins are slow: flash videos, etc...
* No "copy-paste" between Windows and Linux
More info and screenshots: