Is the HDD where GW is your Boot drive, or a satelite drive?
If you have a second HDD, try outsourcing files (Movies, or documents for example) to that one. Dont try to move any files for installed programms, as that would screw up the registry.
Also, empty your Recycle bin and the Cookies/Browser history. That should give you a few GB worth of space if you havent done that in ages.
After you make more space, try to dowload again.