It's been awhile since I installed BT as dual-boot, so my memory is a bit rusty. I don't recall seeing a side-by-side option, as such. I think the default install is for dual-boot. You should see a slider bar for /dev/sda2 (as that looks to be your primary partition). Try adjusting that to suit your needs, and continue with the install, per the wiki directions. Also, like the wiki says, be sure to back up your Windows partition first!