Well at least it wasn't a problem on your end.
For those who do want to run Steam-restricted games on XP because the game works better on XP (or doesn't work at all on anything newer), there is some discussion of the issue over at VOGONS.https://www.vogons.org/viewtopic.php?f=5&t=60670
Some people still don't realize that by default Steam's "offline mode" lasts less than 2 weeks before it requires a connection to the Internet again. There is some discussion of workarounds in that topic.
IMO Steam should make these games DRM-free -- the games they sell that only really run reliably on XP should not have to connect to the Internet.