Switching default hard drives
So I just bought a new laptop 3 or 4 days ago I think. It runs Windows 7 Home Premium 64-bit. So its specs included a "1TB HDD" (500GB+500GB 7200RPM). Under the 'Computer' in Windows 7, it shows that there are 4 hard drives (or they could be separate partitions of the 2 hard drives for all I know), which add up the memory to around 950 or 900GB, which is agreeable.
My issue here is, I've already used up the first C: drive completely, and now Windows is prompting me constantly to free up space so that I may continue storing data on it. But there are 3 free hard drives which I do not know how to make Windows use. It's brand new, so I doubt that it was because of something I did wrong. I will attach a picture to show what I mean more clearly.