Stolen thoughts, the problem is that looking at C: drive properties(which is telling you 28 GB used and 1 GB free) is different than selecting all files/folders inside C:\ drive and looking at the combined property of those because there are so many hidden things that do not show up when you explore C:\ drive. Read the article below, it should explain that in a bit detail.
Disk Space Discrepancy
According to that article, the C: drive properties gives you the accurate reading not selecting C:\ drive folders and checking their properties. So you might actually have only 1 GB space left. Might wanna start getting rid of some of the files.