I think I've heard of something similiar... there is some "ad-ware" that downloads itself to your 'puter, then sits and monitors your http requests and redirects them to advertiser sites. There is software available to detect and remove these annoying things, but I don't recall the name of it... I'll do some looking around in Google and get back to you.
Haven't used this particular bit of software myself, but from the description it seems very complete.