|You probably have only one physical drive but it is partitioned into two separate partitionswhich show in windows with different drive letters.|
"i have no ideal why i have two hard drives and no one has heard of it either"
Correction. Everyone (But you) has heard of employing separate partitions on hard drives. Someone created two partitions on your drive. One that contains your operating system and the second for use as storage. The fact that the second is empty means you simply have not used it yet.
If the System drive ( The one with Vista on it) is getting full then you should consider moving some files to the other drive.
Since you appear to lack experience I would be reluctant to point you towards other options that would involve deleting the un-used partition and then expanding the Vista partition into the un-used space. There are inherent dangers when you start messing about with re-sizing operating system partitions and you could easily wind up with a brick
Goin' Fishin' (Some day)