In fact, unallocated space cannot be deleted but can be well-used. Just allocate it to other partition or create a new partition on it.
Right-click on the unallocated space and check what you can do about it in Disk Management. There’s New Simple Volume option in the menu, if you don’t want to create partition on it, or cannot create the fifth partition on the disk, you can check if the some partition on the disk can be extended or not.
Many Computer companies have OEM partition in their computer, they may not well-distributed disk space, like Dell laptop or desktop, many take much free space on C Drive, or reserved large capacity of size even when the hard drive has only small volume. So we can delete OEM partition in Acer, ASUS, Dell, HP, and LENOVO. Etc. We can delete OEM partition using diskpart command line, or use free partition manager software Macrorit Partition Expert.
Window.old folder is here for Downgrade or rollback to previous Windows Version if you don’t prefer the Newest Windows 10, so Microsoft time-scheduled this for one month, after when the Rollback option won’t work, thus you don’t have to worry about the size of the Windows.old folder, it will be deleted after 30 days, but if you want to delete Windows.old manually, we have many ways to do.
Format is the last thing to do when we have no choice on solving disk partition low disk space problem on C drive. But recent years, especially the year when Windows 7 released, personal computer industry became thrive. Disk management software had their days back again. Nowadays, with the new Windows 10 installed, the Format operation is not the first option came up in our mind. We had better solutions.