|If you have extra space/separate partition/volume on your HDD, then you can increase/decrease the space used for the system drive without reformatting. Open compmgmt.msc and then click Disk Management on the left pane. Compmgmt.msc can be found in Control Panel\System and Security\Administrative Tools. You must have admin privileges to run compmgmt.msc.|
Find the partition/volume you want to add to the system drive.
First, you must delete the partition/volume which you want to add to the system drive. This partition/volume must be adjacent to the system drive you want to expand. If it is not, then you cannot expand the system drive. So be careful to choose the one that is adjacent to the system drive especially if you have more than one extra partition/volume present on the HDD.
Do not forget to copy the contents of the partition/volume you want to delete to a safe place such as an USB flash disk before you actually delete it.