Right click "My Computer" and Select "Manage"
In the left pane under "Storage" select "Disk Management"
Now u will get the system whole partition detail in a pane below.
Blue represents the Primary Partition i.e OS drive
The Unallocated space in ur hard disk will be shown black color
Right click the unallocated space and select " New Simple Volume" option
Now follow the steps and allocate the amount of space that u want.