I have some problem installing windows XP, on my C disk I have 20 gb, I want to reinstall my windows XP and extend the volume of the C disk to 30 gb (I want to take free space from my D disk without touching information on it), is it possible, how can I do that, thanks in advance
From what you've said I assume you're dual-booting. So I assume that unless you have 10GB free space on your hdd you're going be decreasing the size of another partition, presumably with another OS installed. Whenever you want to resize a partition you risk losing everything so first of all I would back up everything, on all partitions, to an external hdd.
Head over to http://gparted.sourceforge.net/ and download the Live CD .iso and burn it to a CD. Pop it in your drive, restart and boot into GParted.
From there you can resize your partition.
I do stress though that even though GParted is excellent at non-destructively editing partitions its still not an operation that you want to undertake without backing up everything first. So you do so at you're own risk.