1. You would use Windows’ built-in Disk Manager. To access that, click Start, Right click Computer and select Manage
2. From left hand column, expand Storage followed by Disk Management
3. You’ll ee your hard disk statistics. Choose the partition on which you want to install Windows 8. You will be given a choice for either shrinking or expanding the partition. (PS: You should have at least 20 GB free for an error free installation of Windows 8). Right click on the main drive and select Shrink Volume from the drop down list. Now type in the minimum space you want to allocate to a new partition drive. Make sure it is more than 20 GB for Windows 8 Consumer Preview to run effectively.
5. Once you’ve followed the steps above, download Windows 8 Release Preview from this website:
When you run the setup, let it initilize, and select Install on another partition
6. From here, Windows will automatically run computer’s compatibility tests to see that the new installation can work effectively & efficiently on the new partition