I suddenly have a lot more adverts. Help?

[Anna]

Yesterday I clicked on a trusted link to view a funny Potter pic. I was told that I had to download some Java thing, which I did, because my laptop said it was a verified publisher. Anyway, I had a bit of trouble with some other search engine called snapdo replacing google which I was able to sort out, but now every site I go on has adverts at the bottom and the side that it never had before! I wasn't sure until I saw all the new ads on Youtube, and it's really annoying because they keep speaking! Any idea how I can sort this out? Thanks.