If you have a spare disk, cloning first.

Clonezilla is a good option if you have some skills. You can put it on a thumb drive and boot from usb if your comp can.

then there's Gparted.

If you want payware, partitionmagic is a solution, I think there's an old version that is free, I don't know if it could have pbs with XP SP3.
Anyways, cloning is a first so that you have a carbon copy of the drive before anything wrong could happen.