When I installed Vista, I choosed to have 30 gb for C (system, installed
programs etc.) and the rest for D. However, 30 gb was not enough but I can't
move space from D to C.

I've choosen Start, than Computer (I have the swedish one but I'll try to
translate), right click and Edit. I choose disk and right click on D and
shrinks it 10 gb. Now I can give it more or less space. But I can only shrink
C, more space is grey even when I have 30 gb of free, non used space. Where's
the catch?