QtParted is a nice front-end to parted, but you still have to be careful. I've used these tools maybe six times, and five times everything went perfectly. Once it didn't, however. Thanks to fsck and some fairly recent backups, I didn't fare too badly, but be forewarned. Back up as much as you can.
I think it's a good idea to run QtParted from the System Rescue CD. That way you know for sure that you haven't accidentally mounted one of the partitions that you're trying to resize. I find the System Rescue CD good to have on hand, anyway. You can find an iso at http://www.sysresccd.org/.
When you boot with the SysResc CD, you'll get instructions on-screen about how to run QtParted.
Wiser heads don't even use these tools, but for many of us, it's hard to resist. I'm using my WindowsXP installation less and less often, and I'm always wanting more space for Slack, so I just recently resized my WinXP partition (NTFS) using QtParted. That gave me a nice little chunk of disk space for Slackware. Rather than resizing my Slack partition, which I thought was pushing it a little, I just created a new Linux partition where I stash a lot of extra stuff.