[River]

snowmonkey just told you what he thought...that it sounds like a virus.

he then proceeded to give you info to start with to check this out. he listed over 20 links that cover the spectrum of possibilities based on the info you provided. the problem could be a spyware virus, an adware virus, of one variety or another.

if you have ran a anti virus program and it found nothing, first of all is your virus definition file up to date?

I beleive that most of these spyware and adware programs would not be found by a anti virus only program(?)

An adware virus is more than likely the culprit, from probably some site you visited and clicked on some thing of interest and in a lot of cases unknowing to the user, the adware virus program is installed.

follow the suggestions offered by snowmonkey, more than likely will fix your problem, and if not, look at the knowledge you will gain. what have you got to loose?

imho
River~