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I think the risk would be minimal. Yes, the XP machine is still on your network, so there could be a risk, but if you are not connecting to the internet that removes the most likely attack vector. However, it is connected to your network so there is still some risk. Make sure to keep the XP machine behind a hardware firewall (NAT router). Also, follow standard advice such as make sure you got all the latest updates and make sure you have updated antivirus software. Remove administrative rights on the XP machine. You could also configure Windows so that you only connect to the network manually when you need to do so. I don’t think that requires admin rights on XP, could be wrong about that, though.