You are right at the edge of what a HD can do. A defragger needs space to defrag. For that reason, you should always keep about 15% free space available on any harddisk.
Your HD figures as quoted, prove that theory.
Do some spring-cleaning, Temp directories, files with extensions .cpy or .tmp and uninstall those programs/games you have not used since last summer!
Then crack open your piggy-bank and go buy another harddisk, they are cheap.