I would say there is another quite fast option, when installing select the
option to resize the Windows partition (not using Gparted, just the installer).
It will safely resize the Windows partition and then the installation will go
with no problem.
Gparted is damn slow when working with large disks and partitions.
Yes, backing up your files on the Windows partitions is a good idea. Having a
Windows 7 install disk around is also a good idea.
No, really, Ubuntu can be three-booted with FreeDOS (that one wants to stay on
the first, C partition, really) and Windows 7 with no problems.