have you checked firewall?
never used silverlight,
when you re-install and it up dates again, do you see a flash of the blue screen? Im using latest mac pro.
I know it sounds crazy, delete everything, reboot, install agin, check. before you delete again reboot and again check.... personally i think it is a path error.
My problem like yours, i did something..... and it started working again. sorry cant help anymore