Grab a copy of gparted or more likely a copy with a gui front end like QTparted. and go to town.

There are Linux LiveCD's made specifically for repartitioning jobs, such as this one. But Windows Vista and Windows 7 both have a built-in disk management tool called ... "Disk Management." I don't use Windows but you should be able to type that into the Search box after you click the Start button (or whatever they call it these days). Or go to the Control Panel and click on the icon named Administrative Tools or something similar, and it should be there.

Quick reference from Google:

http://www.nirmaltv.com/2009/05/12/how-to-resize-disk-partition-in-windows-7/