How to Allocate Free Space to C Drive in Windows 8

Using Disk Management

1. 
   Go to Disk Management. Right click Computer and select "Manage."
2. 
   Locate the free space under Disk Management. In this case, there are 15 GB that are unallocated.
3. 
   Right click C drive and select the item "Extend Volume".
4. 
   When the Extend Volume Wizard opens, click Next to proceed through the setup.
5. 
   Select the free space you want to allocate and click Add. When you've finished selecting space, click Next.
6. 
   Review your selection and click Finish.
7. 
   Make sure the process worked properly. The 15 GB free space should successfully be allocated to C drive.