I'd appreciate any help I can get with this. 

So..One day I was surfing clothing sites, regular sites like Walmart, Just my size, etc, nothing kinky. I suppose it popped up while I was surfing, but I did not pay attention. Later I noticed it popped up at random several times a day. It opens a new window, and at first it says .adserver.com. (I added the periods to keep it from being a real link) Then it morphs into another site, once it was a search engine. I usually close it when it pops up, so I don't know what other sites it morphs into. What I need to know is, what do I do to block this site? I am a relative beginner when it comes to the inner workings of my pc, so be gentle with me. ( I see no connection with what pages I have open and when it pops up, but I know it does when I use Google search)

Thank you in advance. 

V