Disk partitioning is the act of dividing your HDD into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks (Wikipedia). This post will show you step by step guide how to create a new partition (also called a volume) in your HDD by shrinking an existing partition first.
Click Start > Control Panel
Select "System and Security"
Click "Administrative Tools"
Double click on "Computer Management"
Select "Disk Management"
Right click on drive which you want to shrink, and then click "Shrink Volume"
Enter the amount of space to shrink in MB (Mega Bytes) example: 20000 for 20GB (Giga Bytes), then click Shrink WARNING!: If you want to install any OS in this partition you should stop here. Follow this guide to install Windows 8
Select the new unallocated Volume (left click on it), then right click > select "New Simple Volume"
"Welcome to the New Simple Volume Wizard, This wizard helps you create a simple volume on a disk. A simple volume can only be on a single disk. To continue, click Next." Click Next
"Specifiy Volume Size, Choose a volume size that is between the maximum and minimum sizes." Click Next
"Assign Drive Letter or Path. For easier access, you can assign a drive letter or drive path to your partition." Please choose drive letter. e.g: Z then click Next
"Format Partition To store data on this partition, you must format it first." Choose whether you want to format this volume, and if so, what settings you want to use. Click Next
Completing the New Single Volume Wizard You have successfully completed the New Simple Volume Wizard. Click Finish
"The operation you selected will convert the selected basic disk(s) to dynamic disk(s). If you convert the disk(s) to dynamic, you will not be able to start installed operating systems from any volume on the disk(s) (except the current boot volume). Are you sure want to continue?" Please read "WARNING" on step 7… or click Yes