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I found this on the internet: http://fshistory.simflight.com/fsvault/
You can download the very first FS and some other old versions.
I have to say that I didn't tried it yet so I don't know if it works on XP.
That FS 5.0 (it was flightsimulator 95) was the first ever version I played and I still have it on CD-ROM somewhere! I didn't try yet, but might try to install it once, just to compare!