You rather need to shrink the NTFS partition, install Trisquel alongside to
your Windows and install the Linux bootmanager (is it GRUB or GRUB2 with Tris
?) on your harddisk. Technically it is not hard to do but still can endanger
the data integrity of your Windows installation. Grub can screw up your boot
record, so you have to re-install the Windows boot stuff from the CD in the
recovery console.
Essentially the Trisquel setup tool will guide your through the process. You
simply need to understand the neccessary steps.
I would rather use a second HDD. May be you have a spare old HDD lying around
for Trisquel ?
Or you choose a similar approach as I did and install Trisquel to a USB
stick. I am currently experimenting with Trisquel x64 5.5 on my Windows
gaming system and have installed it to 16GB USB stick. Trisquel did also
recognise my Windows installation. So when booting up from the USB stick I
can choose booting up Windows as well. I the USB stick is too small there are
ways to extend the available diskspace by simple using binary files stored on
your Windows HDD mounted via loop-back but this requires a bit of Linux
skills in the shell.
Trisquel is also a bit picky about the hardware. Esp. NICs on the motherboard
often rely on firmware blobs which are not supported by Trisquel. A cheap USB
NIC can be a nice workaround.
