Unusable hard drive space?

my laptop, an asus, model k...something, has a 300g hard drive. however, im only using about 70g? i go into the my computer thing, and it says i have 2 drives. a c, and d. the C is getting filled up, while the D (which says data) is completely empty. i use my computer for recreation and gaming mostly. can my D drive be opened up to store downloads or anything else i want to put on my computer? or is it specifically used for other things, and is just going to remain empty space...?