on my powerbook my harddrive is divided in 115 gb for osx and 115 gb for windows xp.
When I want to divide that into -let's say - 175 gb for osx and 40 for windows, is that easy to do? or should everything be reinstalled again?
I assume you are using bootcamp. If that is the case just use the boot camp utility to remove the windows partition. Then use it again to create a smaller partition for windows. It wont effect anything on the mac side.
There are no "technical solutions" to your "artistic problems".
Don't let technology get in the way of your creativity!