Hi - I'm assuming you are not using GlobalProtect agents - if you were you could create a HIP object for the XP OS and target your rule that way. Without GP, if your DNS performs well you could create an address group and put the FQDN's of the XP machines in, then do a deny rule to the Internet. Or more preferably to take potentially slow/inaccurate DNS resolution out of the picture, if the XP machines have static IP addresses, or effectively static by DHCP reservation, you could do the same using the IP addresses instead of the FQDN's. Hope this gives you some possible options.
You might be able to create a custom signature for browser user-agent headers. It's no guarantee, but might be enough. The IE string is "Windows NT 5.1", and in other browsers it will probably list "XP". You'll probably want to make it case sensitive and search for strings online to help get a better picture.
Hope this helps,
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