Go to device manager, disk drives and double click on your hard drive then click on settings.. if current drive letter assignments show a c: & d: then you have one hard drive with two partitions. Like the others say.. not a bad thing, its more of a good thing.
If it shows only c: then you have a virtual drive loaded. Check your config.sys file for an entry like subst D: c:\****
If you see that line.. rem it. Then reboot... now you will only have 1 partition.
But its most likely just two partitions. Use that 2nd partition to install important applications, store your music, pics, documents, email etc.