Funny you mentioned that it might be a firefox addon i started this tread saying that there was an infection on firefox addon which showed an add of a company in all of my google searches. There am shearing a link where i have warmed other about it on another forum. Also It was the same time when i made the switch from MSE to avast.

http://www.thinkdigit.com/forum/internet-www/175983-fake-link-appers-google.html

Think this might shead some light.