My router has a pretty descent firewall so i feel like it should be safe? Maybe. Obviously I won’t be signing into any online accounts or anything like that on the XP machine. Is this a bad ideas? Obviously using Windows in general is a bad idea but I’m sort of limited by the hardware here.
The risk is your computer reaching out to something on the internet (either by you browsing, or some automatic background activity) and whatever it finds infects it.
This could be something like an ad on an otherwise trustworthy website.