I set up my dual-boot computer with Win7/Mint 13 using the Mint installer. It worked perfectly. Nevertheless, it's always a good idea to back up any important data.
Many of the partition editors included in Linux installation routines are the same partition editor used in GPartEd. The issue with Debian is may have to do with its not including NTFS drivers by default; it doesn't in the distro and may not on the install disk (I'm just not sure). Debian is aggressively free in its choices for defaults.
GPartEd is a very reliable tool. You should be able to resize the drive with it (or with another distro's installer), then install Debian to the now-unallocated space. Note that my knowledge is most theoretical--I've only ever set up two dual boot computers (Windows/Mint, Fedora/Slackware).
You might get a kick out of the Debian wiki item on NTFS: http://wiki.debian.org/NTFS