The free world's clone of partition magic is called QTParted (from QT a GUI - graphical user interface language - and PARTition EDitor)
The QTParted I usually use is the one included with System Rescue CD. SysRecCD is a bootable linux CD for doing system maintenence. You just download an ISO disc image and burn it to a cd and then boot your computer off the CD. It's 123MB.
http://www.sysresccd.org/Main_Page
QTParted has a grapical user interface which makes it very simple to use.
You'll want to run certain disk scans from XP before doing the partion resizing. Look for a QTParted tutorial. shouldnt be too hard to find one.
I've used it quite a few times on lots of different systems with different drive setups and Ive never lost any data. Sometimes It didnt work because of defective disks but it just gave me polite error warnings and didnt do anything destructive.
However make sure you backup anything important to you.
Also if you want a more full featured bootable CD sized Linux that includes the newest QTParted then try Knoppix.
http://www.knoppix.org/
Enjoy.