Try this from command prompt:
defrag c: -b
There are other switches as well such as:
defrag /? ---- this is the help menu!
defrag c: /a /v /f
In your case if hard disk space is the problem then try this:
defrag c: -a -f
When you do this please close all programs that are running. Also,
empty your recycle bin and delete any *.tmp files. You must be logged
in as administrator or user with those rights.
Your system will definitely see marked improvements.
hth