Networking is the most common issue with PC's. There's a lot to play around with. Your fastest solution, is to run system restore to when it was working properly. However, some troubleshooting steps might include:
Reset Internet explorers options, under 'advanced' to all default settings.
Reset your firewall to defaults
Uninstall the network card from device mananger, and restart let windows reinstall it.
If the issue persists, ensure you have no 3rd party firewalls/antivirus programs, they 'filter' your internet and may need to be re-moved and re-installed.
If this fails, do an 'online virus scan', such as housecall don't rely on your 'onboard scanner'.
Finally, if it still doesn't work, restart the pc, tap f8 (before windows starts to load) and you should get a black screen with text called a 'boot menu'. From this list choose 'safe mode with networking'. It is a diagnostic-form of windows, so no sound, big icons, but will test the Internet speed. If this works, we can go from there.