Why is there two hard drives in laptop

It's possible you have 2 drives, but more then likely you have 2 partitions on the one drive, which will show as 2 drives in Explorer. The second partition which would be about 6-10 GB, would be a restore partition and not for installing or saving files on.
When you choose aa download also choose the destination folder, which you can create on your C drive unless you have a larger drive or partition.
So if you make a folder, say on your desktop labelled Downloads, you would tell the download to save it in that folder, once you do this once correctly it will keep using that as the default location untill you choose to change it.