I think you got it wrong. You are not between hardware and software questions, you are between 1 or 2 additional disks. Why else look for a meagre 9 GB when most would have 100's of GB up to several TB's available?
I can't figure out how I can only have 11GB on this 58.5 GB C: partition...
Uh-oh! The "P-word."
Partitions are leftover from way long ago, when OS's could not see large HDD's. The only reason for one today is for dual-boot, and one would be better off with hot-swappable boot disks, for each OS. Partitions will only slow things down. The OS sees these as two HDD's (or more), and will think nothing of calling for data from "each" at the same time. Obviously, with one set of heads, and one set of platters, the heads can ONLY be in one spot at one time, regardless of what the OS thinks.
As Harm suggests, more HDD's should be in your Christmas stocking - or wherever.
If it were me, I'd wipe the entire Vista disk, erase the partitions and do a clean install of Windows 7. I'd then wipe the XP disk and use that as one of the three minimum recommended for editing (OS, Projects, Media).
Thanks for your replies : I tought about adding another HDD and/or doing a clean install. Un either ways (keeping 3 OS) or wiping Vista, I wonder if this Windows 7 *upgrade* (that is aimed to be installed over Vista...) can do a clean install (either on another drive, or on a drive where Vista would have been removed ??)
Doing so then, would using ''Windows Easy Transfer'' help me not to loose any data amd projects of Adobe ?
Easy transfer is far from easy. It can lead to all kinds of problems. The easiest transfer is a fresh install of OS & programs.