I don't think a defragger can help a full hard drive, they need space to store what they're defragging.
delete unused programs
delete all temp files (some get fucking huge, one PC I did this on had a temp file so large it wouldn't fit in the recycle bin!!)
delete the bulk of your cookies and history
you can always buy a 2nd hard drive, there's plenty of instruction out there on how to put it in, do the pins for master and slave, etc