I’ve been upgrading and rebuilding the Windows computers in my house over the holidays and have been using what I think is the most useful distribution of Linux: GParted. I know that every distribution of Linux has this software, but the live CD makes partition sizing and disk formatting dumb simple for Windows users. Just setup your bios to boot to your CD drive first, stick in the GParted live CD and follow the directions.
One word of caution. Don’t resize your Windows boot partitions or you might corrupt your installation. I did this yesterday with my install of Windows Vista and had to run the repair from the install DVD to get things back to normal. For all other partition resizing, GParted does as good a job as Partition Magic and it’s free.