Like someone said, make sure your actual outlets are grounded too. If they were installed more than a couple years ago, they probably aren't. You can purchase a piece of copper wire at your local hardware store and do it yourself, they cost less than $1 I'm pretty sure. Just make sure you cut power to that outlet, unscrew it, wrap the wire around a screw on the outlet, screw the wire into the back of the outlet box, and reattach it and you're all set. The wiring in my house is fairly old, so one day last summer while I was working with an interior remodeling business they let me have some of those grounds and I fixed up all the outlets in my room since I have so many electronics hooked up constantly. You should also change your outlets to the ones with 3 holes for further protection if they are not this way already.