Yep, you've got taken over by something... BTW, if you're using Norton AV, they have admitted to DOING THAT.

I think it's unethical and wrong, but that's me... they're stealing other people's ad space, kwim?
Anyway, you can see something about it here:
http://www.nakedtrutham.com (no nothing icky on this site, it's a marketing website, honest!) scroll down to where it says "IMPORTANT NEWS" in the column on the left and you'll see the info as well as a link to who he recommends. I use PC-Cillin, which is from
www.trendmicro.com
There are also a lot of scumware type things and ezula that does the same thing, iirc.
Hope you are able to get rid of it soon!! I'm sure they're annoying.