If your computer's hard disk has adequate free disk space, you can install a newer version of Windows on a separate partition and keep the earlier version of Windows on your computer. This is called a multiboot or dual-boot configuration. Whenever you start your computer, you can then choose which version of Windows to run. For more information, see Can I have more than one operating system on my computer (multiboot)?
Multibooting requires separate partitions on your computer's hard disk for each operating system. To create and format a partition, see Create and format a hard disk partition.
You must install the older operating system first, and then install the more recent operating system. If you don't (for example, if you install Windows Vista on a computer already running Windows 7), you 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.
Before installing Windows, be sure to disable all antivirus software and back up your files to an external hard disk, a CD, a DVD, a USB flash drive, or a network folder. Also, find your 25-character Windows product key. You can find it on the installation disc holder inside the Windows package—or in a confirmation e‑mail if you purchased and downloaded Windows 7 online. For detailed instructions about installing Windows 7, see Installing and reinstalling Windows 7.
Turn on your computer so that your current version of Windows starts normally, and then do one of the following:
If you downloaded Windows 7, browse to the installation file you downloaded, and then double-click it (often identified as an Application file under the Type column).
If you have a Windows 7 installation disc, insert the disc into your computer. Setup should start automatically. If it doesn't, click the Start button, click Computer, double-click your DVD drive to open the Windows 7 installation disc, and then double-click setup.exe.
If you've downloaded Windows 7 onto a USB flash drive, insert the drive into your computer. Setup should start automatically. If it doesn't, click the Start button, click Computer, double-click the drive, and then double-click setup.exe.
On the Install Windows menu, click Install now.
On the Get important updates for installation page, we recommend getting the latest updates to help ensure a successful installation and to help protect your computer against security threats. You must be connected to the Internet to receive installation updates. This page might not appear if your computer is not connected to the Internet.
On the Please read the license terms page, if you accept the license terms, click I accept the license terms.
On the Which type of installation do you want? page, click Custom.
On the Where do you want to install Windows? page, select the partition or disk where you want to install the new Windows operating system.
Be sure to install Windows on a different partition from the one where the earlier version of Windows is installed.
Click Next to begin the installation. You might see a compatibility report.
For advanced troubleshooting information about startup problems, see Using System Configuration.
For information about how to safely remove Windows 7 from a multiboot configuration, see Uninstall Windows 7 on a multiboot system.