As with previous Windows versions, it's possible withto configure your PC with two or more operating systems, choosing between them via a boot menu of some kind. And this configuration, typically called a dual-boot or multi-boot configuration, comes with just a few gotchas to consider ahead of time.
First, when possible, you should always install the oldest operating system first and the newest OS last. That's because most Windows versions change the boot loader in some ways, and while newer versions understand the older versions, the reverse is not true. Yes, it is technically possible in most cases to mix and match, but installing from oldest to newest is the simplest, and most seamless.
Second, be sure to prepare your PC for the second OS before trying to install it. This entails preparing the disk, if necessary, by partitioning it to accommodate the new OS. You often won't be able to do this from within Setup, especially if both OSes will sit on the same physical disk (but in different partitions).
OK, let's set up dual boot. In this example, I'll assume you have a Windows 7-based PC with a single hard disk, and you would like to add the Windows 8 Consumer Preview to it.
Add a partition for Windows 8
Since Windows 7 is up and running already, you will next need to partition the disk. This is done with the Disk Management utility. The easiest way to find it is to open the Start menu, type disk part, and the tap Enter. This utility will resemble the following:
You will see a Disk 0 with a C:\ drive that represents your Windows 7 install as well as a system reserved area. To add Windows 8, you will need to shrink that C:\ drive, which will create empty space at the end of the disk. Then you will create a new simple volume (i.e. a partition) from that space, and later install Windows 8 to that volume.
So the next step is to right-click the C:\ drive from within Disk Management and choose Shrink Volume from the pop-up menu. After a bit of futzing, you'll be shown a window like the one here:
How you divide the space is up to you, but Microsoft notes you will need 16 GB of free hard disk space for the 32-bit version of the Windows 8 Consumer Preview, or 20 GB for the 64-bit version. Both of these figures are far too low for a useful, usable Windows 8 install however.
Click Shrink when you've determined how much space to put aside for Windows 8. When you do, a new area marked "unallocated" is added to the end of the disk and the C:\ is shrunk accordingly. (Note that this process is non-destructive, so you will not lose any data.)
Now, right-click unallocated space and choose New Simple Volume from the menu. Stepping through the wizard, give it all the available space, assign it a drive letter, and be sure to give it an obvious name, like Windows 8, so you can find it easily during Windows 8 Setup.
Set up Windows 8
When this is complete, you can install Windows 8. You will have to do this from bootable media, either a Setup DVD or USB flash drive, so you can't use the web-based installer (though this installer can actually generate one of those boot devices if needed). The best bet is to create the Setup media, reboot the PC, and be sure to change the boot device order if required to boot from the Windows 8 Setup media.
The manual Windows 8 Setup process hasn't changed a lot since Windows 7, but there are two key parts of this process where you will need to pay attention:
Install type. You will be prompted to choose between Upgrade and Custom install types. You must choose Custom here, since you are not upgrading Windows 7 and are instead clean installing Windows 8 alongside Windows 7.
Where do you want to install Windows? In the next Setup step, you will be asked to pick a partition or hard drive on which to install Windows 8. You cannot install Windows 8 to the same partition or hard drive to which Windows 7 is installed. Instead, you must choose a different partition or hard drive. So choose the partition you created earlier.
Windows 8 will install normally and eventually boot into the new OS.
Choosing between Windows 7 and Windows 8 at boot time
By default, your PC will boot into the most recently-installed OS, which in this case is Windows 8. But it will briefly stop at a new boot menu, which gives you the opportunity to choose the OS at boot time.
Configuring the boot menu
You can configure this boot menu by clicking Change defaults or choose other options.
The following options are available here:
Change the timer. Here, you can change the boot menu timer to a set group of time limits, including 5 minutes, 30 seconds, and 5 seconds.
Choose a default operating system. By default, your PC will boot into the most recently installed OS if the menu timer expires. With this option, you can choose a different Windows version as the default.
Choose other options. This option triggers the Windows 8 recovery environment, which bears further discussion at a later date.
You can also modify this boot menu a bit from within Windows 8 (but not from within Windows 7). And this other interface provides a bit more fine-grained control over the boot options, especially for the boot menu timer.
To access these Windows 8-based settings, mouse into the lower-left corner of the screen and right-click when the Start tip thumbnail appear. Choose Control Panel from the context menu that appears. And then, in the Control Panel window, search for system. (The Search box is automatically selected for you.) In the Control Panel search results list, choose View advanced system settings.
This will open the following window:
If necessary, navigate to the Advanced tab and click Settings under Startup and Recovery. The resulting window lets you configure the default OS to boot into and, more important, the boot timer to any value you prefer.
And really, that's about all there is to it. Go forth and dual boot!