Virtual Hard Disk or VHD acts like a normal hard drive built into your computer. Cool thing is, it comes in a form of a single file so you can make a copy of it and virtually attach it to any other Windows computer.
Some installations or settings changes require you to restart your computer. In most cases you don't need to actually reboot it, you only need to restart the explorer.exe process.
In this day of age, it's very practical to have a Windows 7 installation on your USB key, for many netbooks and devices don't come with CD/DVD drives anymore.
