|It came with DOS and no OS.|
DOS is an OS. That is what the OS stands for. Disk Operating System. A Windows Command Prompt is not DOS.
I doubt very much if a computer of the specification would come with such an ancient OS as DOS so I think there is something you are not telling us hear.
Did you go to computer Management? If the remaining space is unallocated to you need to partition and first it before it can be used.