When setting up partitions, the C: drive must have sufficient space to hold the OS, plus all programs, etc.. Many folks think that creating multiple partitions is the way to go, but if you make the C: drive too small, then you come up results as you are seeing. When I set up a computer and install the OS, I generally, cause the entire drive to be one partition and make it the C: drive. It then gives plenty of space for all things needed.
In your case, since you recently installed Win8, you may want to reinstall the OS using the installation DVD and during setup, allow all the unallocated space to be formatted.
Hope this helps.
I have just installed Windows 8 and have no programs on the computer but I am told that C drive is running out of space.
When I go to disk management it tells me that there is 142GB of unallocated space.
What can I do?