If your computer's hard disk has adequate free disk space, you can install more than one operating system, and then choose which one to use when you start your computer. This is called a multiboot or dual-boot configuration, and it requires partitions on your hard disk. For more information, see Install more than one operating system (multiboot).
You might decide to create a multiboot configuration if you have programs or devices that only work with older versions of Windows. When you want to use one of those programs or devices, restart your computer, and then choose the operating system that you want to use. Multibooting can also be useful for testing programs or devices that work on Windows Vista for compatibility with Windows 7.
Upgrading replaces your old Windows operating system with a newer version. Setting up a multiboot system installs another operating system alongside your current operating system so that you can use either operating system.
Multibooting requires separate partitions on your computer's hard disk for each operating system. If you're running one operating system, you can't run a program installed on another operating system. You'll need to install programs on each operating system that you want to use them in. You must restart your computer each time you want to switch to a different operating system.
You also need to install the oldest operating system first, which can be frustrating if you want to add an earlier operating system (for example, you're currently running Windows 7, but you want to add Windows Vista or Windows XP).
Installing an earlier version of Windows after a more recent version is already installed can render your system inoperable. This can happen because earlier versions of Windows don't recognize the startup files used in more recent versions of Windows and can overwrite them. For more information, see Install more than one operating system (multiboot).
If you decide that you want to remove Windows 7 from a multiboot configuration, use caution. It can be done safely, but depends on how the computer is set up. For more information, see Uninstall Windows 7 on a multiboot system.
For advanced troubleshooting information about startup problems, see Using System Configuration.