Sometimes malware can change various settings to prevent the computer from accessing the internet properly. First confirm that Internet Explorer is running in Online mode. Start -> Run (Search in Vista/7), then copy/paste the following line into the field and Press ‘OK’.
Copy/Paste: REG ADD “HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings” /v GlobalUserOffline /t REG_DWORD /d 0 /f
Next, find the Network Connections in your Control Panel (you may have to access this from your Network and Sharing Center and then selecting “change adapter settings”.) Find your active connection (it may be a wireless adapter or a physically plugged in LAN adapter, whichever one says “connected”. Right click the connected adapter and select properties. Then find your TCP/IP v4, highlight it and click properties.
Typically, both settings should be “obtain automatically”. If there are numbers written there, write them down and then click on “obtain automatically” and see if you can update. If that doesn’t work you can add the manual addresses back in.
If this does not solve the issue, please refer to the following help question for adding an exclusion for your Anti-Virus/Security/Firewall application.