I haven't seen any free trials of partitioning software. You have options here though. If you no longer want Vista, delete it and use the partition for storage. You could even create a mounted volume to act as a virtual extension of the XP volume. Or use the $80 that you would spend on Partition Magic to buy another hard drive.
I recently installed the RC1 version of windows vista and have begun to run out of space on my xp side of the partition. I have heard that I need partition software that will enable me to delete the vista partition and regain all of my storage space for xp. I was just wondering if there were any free software programs that could do this for me.