You don't say much about your system, so we can only guess, but here are a couple;

1] Proxy server. Open IE, goto Tools > Internet Options > Connections tab. Click the LAN Settings button and make sure there are no "Proxy Settings". If there are, remove the ticks and then tick the "Automatically detect settings".

2] Add-ons. if the above fails, look at my post below about Add-ons;