Storage capacity wrong

I was attempting to save an Adobe Illustrator file to my hard drive, which was about 1.5 MB, and an alert popped up telling me that there was not enough free space available. It was surprising considering I did not have that much on the hd. I experimented by pulling off all the files I already held on the hd and opened the hd's info, which told me I had only 90 KB of free space. I then tried to copy one of the 1.5 MB adobe illustrator back on, and the alert popped up again. At this point, I have no files on the hd and it says it has 128 KB of free space available. Is there anything I can do to work out this bug?