Architecture of web game

I would like to create web game using Virtual Earth. On map it will be vehicles, which position will be counting non-stop and saving to database. Client will be geting new positions of vehicles every seconds. Because of position must be counting non-stop I want create Windows Service, which will be do it and it have store additional data which will be unavailable in database. Client will be using webservice. But I have one problem. When user eg buy new car then webservice must notify Windows Service. How should I do it? Any suggestions about "architecture"?

I want to use Silverlight as client.
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