Why would you do this? Perhaps Windows is so messed up that it fails to boot on its own. Or you want to scan for malware in a clean environment. Or perhaps you just want to play with another operating system without bothering to install it.
The following terms and definitions will help you understand booting from media that doesn't stay inside your PC.
Boot disc: Any bootable CD or DVD. Although not technically accurate, the term is also used for bootable flash drives.
.iso file: A CD or DVD image file that you can download and burn to a disc. To burn one in Windows 7, simply double-click the file and follow the prompts. In Windows 8, right-click the file and select Burn disc image. For earlier versions of Windows, you'll need to install a third-party program such as the free ISO Recorder.
Universal USB Installer: A free program from Pendrivelinux.com that can prepare a bootable flash drive from an .iso file. This doesn't work with every .iso file in existence, but it supports an amazingly large selection of them.
Windows System Repair Disc: This Microsoft boot disc provides several tools for repairing a bad Windows installation. To create this disc in Windows 7, click Start, typesystem repair disc, and press ENTER. Windows 8 provides different options for a CD or a flash drive. For a CD, press WINKEY+R, type recdisc, and press ENTER. For a flash drive, press WINKEY+R, type control panel and press ENTER. Type the word recoveryin the Search Control Panel field in the upper-right corner. Click Create a recovery drive.
Live Linux: This generic term refers to several versions of Linux that can be booted from a disc or flash drive. The most popular of these, and probably the most powerful, is Ubuntu.