Defrag-no room 2drives.

Not new to computers,but seems i am dumb-a-- about stuff never had to do
before.I have XP sp2,Drive C: has only 12% free Drive D:99%free space. So
don't have enough space to even defrag.have done all the remove
cookies,etc.Tells me to remove files i no longer use or are unneeded.What
constitutes unneeded,How do I know,and where are they? I mean I go to all
that and when I bring up the files on the drive it just looks like a bunch of
gobbly-gook to me.
And why is everything on Crive and nothing on Drive. Can I move files
one to another,and which ones are safe to do so? And yes I have read the
books,but just don't seem to be getting it right.Usually when I get answers
from these news groups it always works,and makes it a OH Yeah that's how you
do it moment.
Thanks to all of you,who help all of us who are not tech savvy.