How can I be sure someone is not using my wireless connection?

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Recently my internet has been running really slow (my computer speed is fine). I can just imagine some weirdo neighbour sat at home watching goat porn through my wireless connection (perhaps slightly too graphic, I know). Anyway, I need some tips on how to find out if anyone is using my connection and how I can prevent them from doing so.

I am using Windows Vista and have a netgear wireless adapter WPA.

Thanks.