If you really don't want to lose your windows partition that is fine.
If you have the money you can buy a clean HDD or SDD and put trisquel there.I kinda don't want you to because I see you have 2 HDD and one SSD and space more than enough.
But you said those disks are important and I understand and respect that. So at the end I would incline for you to buy a new disk. That way you won't risk losing any data and can have a safe trisquel installation.
