jachkt, let me try to give you some further details. Usually, PCs come with one harddrive - especially laptops. But desktops can have more than 1 harddrive. So let's say the base case is a system with 1 harddrive. The manufacturers usually divide this harddrive into 2 partition. Partition C: is for the operating system (in your case Vista) and for the user files (documents, pictures, music, etc.). Partition D: is where they store a Vista installation and recovery disk image. Reason is because they are to cheap to give you an installation and recovery DVD when you buy your PC. You are supposed to burn this DVD (in fact you need 2 DVD-R's) form this recovery partition. I highly recommend you do that asap. Type RECOVERY into the start/search field and and the recovery disk creation wizard should show up above. Note, however, that not all makes work the same. Dell for example, ships this DVD with some systems.
Once you burnt your recovery DVDs form partition D:, you could erase everything there and use the space for your own data. It is, however, recommended, not to do that because a) you can never be sure whether your burnt DVDs are any good and b) it it easier to reinstall the system from the D: partition should that ever become neccessary.
If you want further detail, you have to give me the make of your PC and whether it is a laptop or desktop, tell me what is on partition D: (double click on it in Computer) and maybe post a snip of your Disk Management window.