The one gripe I have with Steam right now, is it tries to launch every time activate my computer, and since it takes my computer 5-10 minutes to gain Internet access it always causes Steam to go wanky, and I have to manually cance all Steam processes from the task manager Before I launch a game, or it won't launch.
It is possible to shut that function off, try this:
Open up the main Steam window and select 'Settings' from the 'Steam' menu, then select the 'Interface' tab and uncheck "Run Steam when my computer starts". This will prevent Steam from starting with your computer.
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