Microsoft is connecting a particular product code (copy number) of your FSX
to your particular computer identifier.

Their website is ambiguous but my reading of it is that they will NOT allow
a purchaser to run it on two computers.
I sent mine back to Amazon - they paid the return postage.

I don't think it will be time to buy FSX until next Spring when we know
more about the playability of it and Vista equipped computers with decent
graphic accelerators appear on the market. Maybe then a hack will be
available - and vouched for by users.
Blue