Virtual Memory

So according to My Computer, my C drive's got some 48 gigs left in it to be filled.  However, whenever I download anything nearing an hourlong movie these days, it goes crazy and warns me that my memory's low.  I know that there's a new stick in there somewhere that gave me more than I used to have, but if I remember correctly there was partitioning involved when it was put in, in preperation for something that never happened.  How do I tell my computer to ignore the partition and let me use that extra memory for useful things, like porn and music?

Memory != disk space