Learn how to install free virtualization products like VMware player or Sun VirtualBox in Ubuntu, so you can run Windows, Fedora, or any other operating system on top of Ubuntu in an isolated Window.

Install Sun VirtualBox on Ubuntu 9.04
Learn how to Install Sun VirtualBox on Ubuntu 9.04, so you can run Windows XP, Fedora, and other OS within Ubuntu without the need to restart, repartition hard drives, or risk losing your files.

Install and use VMware on Ubuntu 9.04
Learn how to install VMware Player on Ubuntu 9.04 with the .bundle install file in Terminal Command Prompt, and learn how to install and run a Virtual Machine in VMware with .vmx virtual machine files for VMware Player.

Installing and uninstalling programs with Ubuntu Software Centre for Ubuntu 9.10 (I teach you how to install Virtual Machine Manager in this tutorial)
Learn how to install and uninstall programs in Ubuntu with Ubuntu Software Centre for Ubuntu 9.10.