On the desktop and laptop, I've used GPartEd (on Ubuntu Live CD) and QtPartEd (on a Kanotix Live CD) and never had any corruption. It is able to resize the partition without corrupting data. I've used it at least 20 times, probably more. (Trying various linux distros (Mandrake, Knoppix, Turbo Linux, Slack, BeOS, Ubuntu, RedHat, Suse, OpenSuse, Kanotix, and which one was it that let you play Tetris while it installed?) on both my laptop, and desktop computer.) When I've installed a new hard drive, I used the "Restore CD" that came with my laptop, then resized the partition, and used freed space to install Linux to have a dual boot machine. I bought an extra hard drive caddy, and had one hard drive for daily use, and another for trying out distros. I've trashed my computer plenty of times trying new stuff, (just trashed my C1000 recently trying new stuff) but have never had QtPartEd cause a problem.
OT:
IMHO, fooling with the boot loader (especially GRUB) has always been the thing that was most prone to problems. LILO was always less risky. You could simply uninstall it (lilo -u), and do "fdisk /mbr" or "fixmbr" in Windows repair console.