How do you connect to the internet? Are you behind any institutional firewall as in a University for instance?
Open Internet Explorer (whether it's your default browser or not)
Go to Tools/Internet Options/Advanced tab
If IE6 click "Restore defaults", Apply and OK, if IE7 or 8 click Reset, Apply and OK.
Close and reopen I.E. and go to Tools/Manage Add-ons and re-enable the add-ons.