Redirects in search using firefox

[#1]

I use Firefox 3.6.2 for a browser and when using Google search keep getting re-directs, mainly to advertising, I have redirect warn checked in the browser but on this nothing happens. Reading some things on this suggests it might be to do with DNS and my router and suggests talking to my ISP.
Anyone shed any light or help with this very annoying problem.

[#2]

First thing is to run a couple of programs to look for nasties
http://www.malwarebytes.org/products/malwarebytes_free
http://www.superantispyware.com/
Why are you using an old and insecure version of Firefox

[#3]

Thanks, I am running those programmes will come back whenI have checked both.
I run the Firefox, because I tried upgrading and did not get on with it.
I do use Norton 360, which I thought would find most problems.
As a DMOZ editor I do visit some odd sites.