Create a multiboot startup

If your computer already has any version of Windows installed and you have a second disk partition available (or enough unallocated space to create a second partition), you can install a clean copy of Windows 7 without disturbing your existing Windows installation. At boot time, you choose your Windows version from a startup menu. Although this is typically called a dual-boot system, it’s more accurate to call it a multiboot configuration, because you can install multiple copies of Windows or other PC-compatible operating systems. Having the capability to choose your operating system at startup is handy if you have a program or device that simply won’t work under Windows 7. When you need to use the legacy program or device, you can boot into your other Windows version without too much fuss. This capability is also useful for software developers and IT professionals, who need to be able to test how programs work under different operating systems.

For experienced Windows users, installing a second copy of Windows 7 in its own partition can also be helpful as a way to experiment with a potentially problematic program or device driver without compromising a working system. After you finish setting up the second, clean version of Windows 7, you’ll see an additional entry on the startup menu that corresponds to your new installation. (The newly installed version is the default menu choice; it runs automatically if 30 seconds pass and you don’t make a choice.) Experiment with the program or driver and see how well it works. If, after testing thoroughly, you’re satisfied that the program is safe to use, you can add it to the Windows 7 installation you use every day.

The ins and outs of system drive letters

Which drive letter will your clean installation of Windows 7 use? As with previous versions of Windows, the assigned drive letter varies depending on how you start setup. If you currently have a working copy of any Windows version on drive C and you install a clean copy of Windows 7 on a different partition, drive letters are assigned using the following logic:

• If you begin the installation process by booting from the Windows 7 media and choose a partition other than the one containing your current copy of Windows, the new installation uses the drive letter C when you start up. The volume that contains the other Windows installation uses the next available drive letter when you start your new installation of Windows. When you choose the previous Windows installation from the startup menu, it uses the drive letter C, and your new Windows 7 installation is assigned the next available drive letter. In this configuration, you can be certain that your current operating system is always on the C drive, but drive letters assigned to volumes you use for data might shift in unexpected ways.
• If you begin the installation process by running the setup program from within your current version of Windows and use the Custom (Advanced) option to perform a clean installation on a partition that does not have a drive letter assigned to it, each installation will use the drive letter C as well, with the drive letter for other partitions shifting accordingly depending on which choice you made from the Windows boot menu.
• If you begin the installation process by running the setup program from within your current version of Windows and use the Custom (Advanced) option to perform a clean installation on a partition that currently has a drive letter assigned to it, the new installation uses that drive letter. Other volumes maintain their original drive letters when you start your newly installed copy of Windows 7. Thus, if you run setup from within Windows and choose to install a clean copy of Windows on drive E, the system drive for the new installation will be E as well.

There’s no inherent reason to prefer one of these options over the other. If you find comfort in the consistency of knowing that system files and program files are always on the C drive and you don’t want to have to worry about software that is hard-wired to locations on the C drive, you’ll probably want to choose the first or second option. If you prefer the nonstandard but supported option to use drive letters to keep track of which Windows version is running at any given time, you’ll prefer the third option. But any of these configurations should work reliably with any combination of properly written software, hardware, and settings.

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