You would be well advised to retain both partitions and to move your Documents, Downloads, Music, Pictures and Video folders to the D: drive so that the C: drive contains only Vista and your programmes.
With good back up software - like Acronis True Image - you can then take regular backups of each partition to an external hard drive.
If in the future your computer gets corrupted - it is easy to restore just the C: drive without losing any of your data.
If you do not currently have a back up strategy - sooner or later you are going to lose everything when your hard drive fails - as they all do eventually.
Used properly, Vista is an excellent operating system if you have a computer with the necessary hardware power to run it.