Hi
0) first schedule chkdsk scan of c:
0a) run Disk Management to see how space is allocated on hard drive
If free space is only a GB or so ignore it
1). The free space must be immediately after C: drive
2) you need to boot up off a bootable usb or cd disk with suitable software
As windows built in disk management won't run on active Windows disk
I would use a gparted ISO image burnt to a cd
It could be put onto a usb memory stick
Although this ISO is a Linux disk in most cases it boots up to a windows like interface then starts up the partition editor software
There is free partition management software from a number of other companies such as paragon, easeus etc most provide a Linux boot disk
Regards
Mike Barnes