Sounds like you've got some crapware installed.
Offers toolbars, etc.
I see this kind of thing quite regularly with ads sometimes being injected into pages that contain no advertising.
If you don't actively use the functionality provded by the toolbar, get rid and if you do, well, there no such thing as a free lunch.
To be fair, the advertisers may not be aware of their ads being distributed in this way and may not be happy about it.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale) => Xilo (TT Wholesale due to O2 Wholesale closure) => Zen LLU
Router: Billion 7800N
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.