Click on the download button. Pick a server to download from and pick one of the files that ends with -EN.iso. (EN = English). Burn the ISO to CD and boot from it. Once booted from it, you can find your drive and start deleting or moving files off of your full drive.
If knoppix seems confusing to you, you can google "live linux boot cd" as there are plent of others out there. heck, you may even be able to get away with using UBCD (ultimate boot cd).http://www.ultimatebootcd.com/
Anyhow, do what's easiest for you to understand.