Gparted comes with a bootable CD version, as you can not resize a partition you are using, this is the best solution. Search for gparted, download the bootable CD image, burn a CD and restart your machine using this CD.
Gparted will then allow you to resize your partition to leave room for windows.
I don't know how windows 7 support other system, but (very) old windows installs used to start by re-formatting any disk with another system on it...
So it used to be recommended to first install windows, then install any Linux distrib.