If I plug my windows PC's SSD into the deck can I just tell the deck to boot from that existing windows installation? Or does the drive need to have a fresh installation or something like that
You'd need a fresh install, since the hardware would be very different. I wouldn't recommend even trying, as I've seen folks do similar things with in theory identical hardware and the drive wouldn't boot on the new, and not on the old when moved back.
Is there a way to replace the windows installation on the SSD with a fresh one without erasing any of the data on it? Or would I have to move the data to something else
Not short of backing up the data on another drive. Unless you want to futz with the most core basic drivers that windows will require in order to fully boot. If you don't know how/want to push drivers via CLI, throwing your porn onto an external and then installing windows on the hardware itself works a jiffy.