Windows Vista and newer have built-in tools that can resize volumes. See our tutorial for that here
Windows XP and earlier don't have the tools built-in, so you have to use a third party partitioning tool. We have a tutorial for that, too.
This tutorial covers shrinking an existing volume to make space for a new second volume, but resizing two existing volumes is substantially similar.
Edited by Andrew, 23 February 2012 - 02:30 PM.