You or someone installed it. Did you check Add/Remove programs in the control panel? Just uninstall it and it should be gone.
I'd like to find a way to block Silverlight completely at the source- sort of like using the hosts file to block web pages. Not OS specific, just some completely viscious way so I never-ever see that pop-up, pop-over, pop-under again. I can go to Firefox or Chrome, but still, sometimes I've got to use IE8 and I dislike Silverlight in the strongest terms. TIA