Getting ads with Google results?

So for the past month or so, when I use Google and try to follow the link to the first few results I actually get sent to an ad page, sometimes it's just blank, other times it isn't. I don't blame Google, my friend actually told me that it was adware or spyware or some such. Now, I already have AVG (antivirus software) but obviously it isn't blocking the problem. Any recommendation on what I should do?