Most modern computers seem to come equipped with at least 250G, often more. Since you're running Win8, I'm assuming the computer isn't all that old.

Click on My Computer and see how big the drive is and how much free space is left. If it seems way out of line, maybe some sort of virus is slowly consuming the free space on the drive. If it seems normal, and you seem to have a lot of free space left, You've got another problem (which I'm not sure what it is)

If you have an SSD, it might be quite small (compared with the spinning platter variety) and you may just have to get a bigger drive.