You do not need to reboot in order to switch between Ubuntu and Windows.It can be a dynamically resized virtual hard drive. The size of the installation doesn't have to be predetermined.Installing Ubuntu on a virtual machine inside of Windows has a lot advantages over a dual-boot (but also a few disadvantages).
A dual-boot allows you, at boot time, to decide which operating system you want to use. Many websites (including the one you're reading) have tutorials on setting up dual-boots between Windows and Ubuntu. Please be aware that you should have a minimum of 512 MB of RAM. VirtualBox allows you to run an entire operating system inside another operating system. Actually, you can install pretty much any Linux distribution this way. The screenshots in this tutorial use Ubuntu 12.04, but the same principles apply also to Ubuntu 12.10, 11.10, 10.04, and any future version of Ubuntu. This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Installing Ubuntu inside Windows using VirtualBox You may also want to consider donating directly to the Ubuntu project. If you've found these tutorials useful, please consider donating a small amount to one of my favorite non-profits/charities.