You will need to install Windows 8.1 Preview
to a partition that is at least 20 GB and is separate from your
existing operating system. If you need to create a separate partition
on your hard drive, do the following:
A. On the Start screen, type Computer, right-click Computer, click Advanced, click Manage, and then click Disk Management.
B. Find your system hard drive in the bottom pane.
C. Right-click your system drive and then click Shrink Volume.
Shrink it until you have at least 20 GB of space left on the end of the
drive. (If you already have 20 GB of unallocated space, you can skip to
the next step.)
D. Click the “Unallocated” block on that drive and click New Simple Volume.
E. Follow the wizard through the next few screens, such as assigning it a drive letter. For the Volumelabel, choose a label you’ll recognize like “Windows 8.1 Preview”.