A full backup of your hard disk is strongly recommended prior changing the partition size (even with a 3d-party utility, e.g. Ranish Partition Manager, PowerQuest Partition Magic, etc.)
Even world-wide known hardware/software specialists reportedin their famous books or articles, that at least once their partition management software failed, and only backup saved their data.
So, backup is a must!
The cheapest and fastest backup device today is another harddrive.
I use the embedded Windows backup feature to make a backup on a spare harddrive (actually, just some empty space on another machine's harddrive was enough).
Then I repartition and reformat my initial harddrive the way I desire, and restore the content of my partition(s) from the backup.
As for 3d-party utilities, they might create additional problems, especially on disks with multiple boot.
But I am very happy with Norton Ghost, and I also use it to create an image of my partition(s) on a spare harddrivebefore re-partitioning my harddrives. Then I restore my partition(s) from Ghost image.
Even just simple Windows copy back and force worked flawlessly for me on Windows 2000 Advanced Server and XP (but this is not recommended way of backup, and there are articles explaining why).