Until someone that knows a lot more than me, this is where I would start looking. If you haven't already, run your antivirus in Safe Mode. I am still a fan of Spybot and always have more than one updated antivirus program.
In the Control Panel, look for Device Manager. Look for anything that has a little yellow triangle with !. If you see that, right click and choose Properties. Read what it says.
Another place to check are Services. Something may have stopped running that is necessary for internet connection. Go to Control Panel > Administrative Tools > Services. When you highlight, it should show a description of what that selection is or does.