Just to head off further replies on this thread - I believe the problem was that I was forced to register with microsoft's games for windows (or some such name) when I installed GTA4 many months ago, and it seems that Microsoft have emailed everyone on there, rather than just xbox platform owners. So although unwanted, it was probably legit and just an oversight on their part. Ian Batterbee wrote:

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Has anyone else received spam from Microsoft about an XBox service update ?