I didn't do anything

Guest network on a separate VLAN is the way to go.

This is the way.

I see we are still living in 1999.

my joints are telling me otherwise
- I went on a trampoline with my kid and my knees are still shaking 2 days later....

I ain't no IT expert but what are some things a vulernable windows computer can do?

In general, a compromised system may be running any software the attacker might find useful, including, but not limited to:
keyloggers to find passwords that are in use in the company
software to copy sensitive files to a remote server
software to encrypt the system itself or (if the computer has access to other machines on the network) other computers
produce documents (think, mail) that purport to have been created by the user of the corrupted machine.
The users tend to be less tech savvy than Linux users so they tend to not have adblockers and or allow arbitrary JavaScript from any page to run and or they are running trojanized software because the uploader was "trusted".
Due to market share they are the biggest target.
Untrusted devices should be on an isolated subnet or if you have the time only devices that need to talk to each other should be on the same subnet.
- In an ecosystem where the solution to every problem is “Download this piece of software someone wrote because the standard Windows utilities are worse than useless and don’t provide this basic functionality”, you can’t really blame the users for running every script they encounter uncritically.
Windows PCs often carry viruses that can try to compromise other parts of your network. They are also a privacy concern in a big way especially after windows 10.

Well block all Microsoft website on your routers firewall

Peace was never an option.
Redirect to https://www.wikihow.com/Move-from-Windows-to-Linux

Well I don't see what the problem is…

You must have winshit nightmares.

Call me dumb, but... to me that just sounds like an insecure network, labeled "secure" as long as someone physically guards all the ethernet ports

it probably wasn't very secure network if anyone can join and cause this. so instead of being misogynistic moron, you could spend some more time on improving your security.

So now it’s misogynistic to identify a girl on a picture or an unnamed person in an anecdote as “she”?
- no, it is misogynistic to draw a picture where she is the incompetent user causing problem and he is the brave security engineer whom she has to beg for forgiveness.
  when it is in fact his shitty security that is root of the problem, because users will be users, no matter what gender. so it is the "hero" of this magnum opus who is really the incompetent one. do you understand how laughingly pathetic that is? bragging about your "secure network" when one user with windows can burn it to the ground?